Remote measurement of shallow depths in semi-transparent media

ABSTRACT

Through discrimination of the scattered signal polarization state, a lidar system measures a distance through semi-transparent media by the reception of single or multiple scattered signals from a scattering medium. Combined and overlapped single or multiple scattered light signals from the medium can be separated by exploiting varying polarization characteristics. This removes the traditional laser and detector pulse width limitations that determine the system&#39;s operational bandwidth, translating relative depth measurements into the conditions of two surface timing measurements and achieving sub-pulse width resolution.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation-in-part of U.S. patentapplication Ser. No. 15/092,015, filed Apr. 6, 2016, which claims thebenefits of and priority, under 35 U.S.C. § 119(e), to U.S. ProvisionalApplication Ser. No. 62/143,502, filed Apr. 6, 2015, which is acontinuation-in-part of U.S. patent application Ser. No. 14/129,925,filed Dec. 27, 2013, now U.S. Pat. No. 9,476,980, which is the NationalStage of International Application No. PCT/US2012/045038, filed Jun. 29,2012, which claims the benefits of and priority, under 35 U.S.C. §119(e) to U.S. Provisional Application Ser. No. 61/503,314, filed Jun.30, 2011, the present application further claims the benefits of andpriority, under 35 U.S.C. § 119(e), to U.S. Provisional Application Ser.No. 62/309,163, filed Mar. 16, 2016, all of the above-mentioneddocuments are fully incorporated herein by reference in their entirety.

FIELD

This invention is directed generally to methods and systems for remotesensing measurements, and particularly to methods and systems for remotesensing measurements through semi-transparent media, e.g., glass,plastic, sapphire, and combinations thereof, and even more particularlyto methods and systems for sensing a relative distance and/or shape ofan object or feature in various environments.

BACKGROUND

Light detection and ranging (lidar) bathymetry is a technique capable ofmeasuring the depth of a relatively shallow body of water (e.g., lessthan 2 meters). A pulsed laser beam is transmitted from the lidarinstrument to the body of water. The light generated by the laser beamis typically in the blue-green portion of the spectrum due to the hightransmission through water of light at that wavelength. Portions of thelaser pulse scatter from the air/water interface, the water volume, andthe floor of the water body back to and are collected by the instrument.The times of flight of the detected signals are converted into rangemeasurements and, upon consideration of viewing geometry, propagationpaths, and associated errors, permit determination of the probed waterdepth.

Depth measurement in the shallow water regime is challenging due tosystem bandwidth limitations of traditional bathymetric lidartechniques. Traditionally the approach used to resolve two scatteringobjects separated in range is by resolving the difference in travel timeof light between the two objects. The limiting factor in resolving therange between targets in traditional systems is dictated by the pulsewidth of either the laser pulse or the detection electronics, oftendefined as cτ/2 n, where c is the speed of light, n is the index ofrefraction of the media, and τ is the limiting pulse width in time (orthe inverse of the system bandwidth). Consequently in shallow depthapplications the limit of current lidar technologies occurs whereambiguities exist between surface scatterings, volume scattering alongthe water column, and floor scattering due to system bandwidthlimitations associated with laser and/or detector pulse widths. As aresult, present day bathymetry lidar systems are limited to depthmeasurements no shallower than tens of centimeters.

There is a need in the art to improve the precision and other aspects ofbathymetry lidar systems.

SUMMARY

Accordingly, the invention is directed to methods and systems for remotemeasurement of shallow depths in semi-transparent media thatsubstantially obviate one or more of the problems due to limitations anddisadvantages of the prior art.

An advantage of the invention is to provide enhanced range resolutionand precise measurement in shallow water depth measurement and waterfloor topography mapping. Further, the invention has capabilities andapplications in semi-transparent media thickness measurement and surfacetopography characterization.

Another advantage is to allow distance sampling with no physical contactwith the media.

Yet another advantage is providing a low cost, accurate,self-calibrating, and scalable solution with a differential measurementrequiring no knowledge of the lidar system's platform vertical position.

Still another advantage of the invention is to provide a method andsystem for navigation of autonomous vehicles.

Yet another advantage of the invention is to provide a method and systemfor navigation through a semi-transparent media.

Another advantage of the invention is to provide enhanced rangeresolution and precise measurement in shallow water depth measurementand water floor topography mapping. Further, the invention hascapabilities and applications in semi-transparent media thicknessmeasurement and surface roughness characterization.

Another advantage is to allow distance sampling with no physical contactwith the media.

Yet another advantage is providing a low cost, accurate,self-calibrating, and scalable solution with a differential measurementto determine depth requiring no knowledge of the lidar system's platformvertical position.

Another advantage is to provide optical properties of a semitransparentliquid by performing reflect transmissometer measurements.

Further yet another advantage is providing a sub-pulse width resolutiontechnique using a lidar system for detecting and characterizing “softtargets” whose optical density becomes sufficiently large to producemultiple scattering effects in the media. For example, measuring waterquality as characterized by a level of turbidity.

Additional features and advantages of the invention will be set forth inthe description which follows, and in part will be apparent from thedescription, or may be learned by practice of the invention. Theobjectives and other advantages of the invention will be realized andattained by the structure particularly pointed out in the writtendescription and claims hereof as well as the appended drawings.

To achieve these and other advantages and in accordance with the purposeof the present invention, as embodied and broadly described, a lidarsystem includes a transmitter configured to output a pulse of polarizedlight to a medium; the transmitter includes a laser and a polarizer inoptical communication with the laser. The lidar system further includesa receiver configured to collect multiple scattered lights from themedium; the multiple scattered lights includes received pulses havingvarying angular spread or depolarization. The lidar system furtherincludes a first detector; the first detector comprising a sensors arrayof light sensors. The lidar system further includes a second detector;the second detector comprising a second sensors array of light sensors;the first detector and the second detector are each configured toreceive at least a respective component of the multiple scattered lightsfrom the receiver; and the light sensors of the first sensors array andthe second sensors array are each configured to receive a respectiveportion of the received pulses. The lidar system further includes timingelectronics coupled to each of the first detector and the seconddetector; an output of the timing electronics comprises a valueindicative of a relative distance based on an amount of time elapsedbetween inner portions of the received pulses of the multiple scatteredlights, thereby achieving a sub-pulse width resolution; optionally thevalue is based on an optical separation of respective component portionsof the multiple scattered lights.

In one embodiment, the transmitter further includes a half-wave plate inoptical communication with the laser. In another aspect, the transmitterfurther includes a prism in optical communication with the laser. Inanother aspect, the laser includes at least one of a polarized laser,pulsed laser, and a continuous wave (CW) laser. In another aspect, thepolarized light includes a known polarization. In another aspect, thepolarized light includes circular polarization. In another aspect, thereceiver includes a telescope. In another aspect, the receiver furtherincludes a spectral filter. In another aspect, the lidar system furtherincludes a polarizing splitter; the first detector and the seconddetector are in optical communication with the polarizing splitter. Inanother aspect, the respective component of the received pulses of themultiple scattered lights comprises a cross-planar polarizationcomponent and a co-planar polarization component. In another aspect, thefirst detector is calibrated to receive substantially the cross-planarcomponent and the second detector is calibrated to receive substantiallythe co-planar component. In another aspect, the first detector and thesecond detector are each calibrated to the respective portion of themultiple scattered lights.

In another embodiment, a lidar system includes a source of polarizedlight configured to output a pulse of polarized light and a lightreceiver configured to receive multiple scattered lights comprisingreceived pulses having varying angular spread or depolarization; thelight receiver comprising a polarizing beam splitter; the polarizingbeam splitter is configured to split the received pulses intocross-planar polarization components and a co-planar polarizationcomponents. The lidar system further includes a first detector includinga sensors array of light sensors; the first detector is configured toreceive the cross-planar polarization components; and the light sensorsof the sensors array are each configured to receive a respectivecomponent of the cross-planar polarization components. The lidar systemfurther includes a second detector including a second sensors array oflight sensors; the second detector is configured to receive theco-planar polarization component; and the light sensors of the secondsensors array are each configured to receive a respective component ofthe co-planar polarization component. The lidar system further includestiming electronics coupled in electrical communication with the firstdetector and the second detector configured to output a value indicativeof a relative distance based on an amount of time elapsed between innerportions of the received pulses of the multiple scattered lights,thereby achieving a sub-pulse width resolution.

In one embodiment, the source of polarized light includes a laser, ahalf-wave plate, and a polarizer. In another aspect, the laser isselected from the group consisting of a polarized laser, pulsed laser,and continuous wave (CW) laser. In another aspect, the first detectorand the second detector each includes a photomultiplier tube, thephotomultiplier tube counts photons in the cross-planar polarizationcomponent and the co-planar polarization component, respectively.

One embodiment is directed towards a method of determiningcharacteristics of at least two surfaces. The method includestransmitting a pulse of polarized energy to at least two surfaces andreceiving reflected energy from the surfaces received over a period oftime after a transmission of the energy. The system further sensesinformation indicative of one or more properties of two or more portionsof received reflected energy. The properties of each of the portions ofthe received reflected energy includes one or more of (a) an orientationof the portion, (b) an angular spread of the portion, (c) a degree ofpolarization of the portion, (d) an azimuthal polarization pattern ofthe portion, (e) a high order scattering profile of the portion, and (f)a range of a respective property being one of the properties (a) to (e).The method also includes determining with computational equipment one ormore elapsed time among the two or more portions of the receivedreflected energy, based on one or more differences among the propertiesof the portions where the elapsed time is less than a duration of thepulse of polarized energy.

Another embodiment is directed towards a method of imaging and profilingthe skin of a patient. The method includes transmitting a pulse ofpolarized light to the skin of the patient and receiving a collection oflight where the collection of light includes light reflected from theskin surface and subcutaneous layers of the patient over a period oftime after a transmission of the pulse. The method also includes sensinginformation indicative of one or more properties of two or more portionsof received light in the collection. The properties of each of theportions of the received light include one or more of: (a) anorientation of the portion, (b) an angular spread of the portion, (c) adegree of polarization of the portion, (d) an azimuthal polarizationpattern of the portion, (e) a high order scattering profile of theportion, and (f) a range of a respective property being one of theproperties (a) to (e). The method also includes determining withcomputational equipment characteristics of the skin, e.g., pathologynormal, cancerous, precancerous and the like, based on one or moredifferences among the properties of the portions, wherein the elapsedtime is less than a duration of the pulse.

Still yet another embodiment is directed towards a navigation system.The navigation system is configured to assist with vehicle operation orprovide autonomous vehicle operation. The navigation system includes alight transmitter configured to transmit a pulse of polarized light toat least two surfaces within a transmission path of the pulse, a lightreceiver configured to receive a collection of light. The collectionincludes light from the surfaces received over a period of time after atransmission of the pulse by the light transmitter and a sensor systemconfigured for sensing information indicative of one or more propertiesof two portions of received light in the collection. The properties ofeach of the portions of the received light includes one or more of: (a)an orientation of the portion, (b) an angular spread of the portion, (c)a degree of polarization of the portion, (d) an azimuthal polarizationpattern of the portion, (e) a high order scattering profile of theportion, and (f) a range of a respective property being one of theproperties (a) to (e). The system also includes computational equipmentconfigured for determining an elapsed time between the two portions ofthe received light, based on a difference between the properties of theportions, and a relative distance based on the elapsed time, wherein theelapsed time is less than a duration of the pulse.

The present disclosure can provide a number of advantages depending onthe particular aspect, embodiment, and/or configuration. These and otheradvantages will be apparent from the disclosure. Additional features andadvantages may be learned by the practice of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this specification, illustrate embodiments of the invention andtogether with the description serve to explain the principles of theinvention.

In the drawings:

FIG. 1 illustrates a lidar system in accordance with embodiments of thepresent invention, in an exemplary operating environment;

FIG. 2A and FIG. 2B illustrate detection of scattered light pulse overtime for a system with pulse width resolution;

FIG. 2C and FIG. 2D illustrate detection of scattered light pulse overtime for a system with sub-pulse width resolution in accordance with anembodiment of the present invention;

FIG. 3 illustrates a lidar system in accordance with an embodiment ofthe present invention;

FIG. 4 depicts components of a lidar system in accordance with anembodiment of the present invention;

FIG. 5 depicts components of a lidar system in accordance with anembodiment of the present invention;

FIG. 6A illustrates the normalized received intensity of light fortargets of varying degrees of depolarization;

FIG. 6B illustrates normalized detector voltage data acquired during thereception of backscattered signals using an analog system for the watersurface (dotted) and floor (solid);

FIG. 6C illustrates normalized timing data acquired during the receptionof scattered surface and floor signals using a digital system for 3centimeter water depth (solid) and 1 centimeter water depth (dashed);

FIG. 7 illustrates the measurement of depth of semi-transparent mediawith sub-pulse width resolution;

FIG. 8 illustrates components of a lidar system according to anembodiment of the invention;

FIG. 9 illustrates the measurement of water quality in terms ofturbidity of semi-transparent media with sub-pulse width resolution;

FIG. 10 illustrates an exemplary block diagram of a communicationnetwork for use with embodiments of the invention;

FIG. 11A illustrates an experimental setup of Example 4 configured tomeasure the height of at least two surfaces in still substantially clearwater;

FIG. 11B illustrates a graphical representation of exemplary results ofExample 4;

FIG. 12A illustrates an experimental setup of Example 5 configured tomeasure the height of at least two surfaces in slow flowing water;

FIG. 12B illustrates an exemplary top down view of experimental setup ofExample 5;

FIG. 12C illustrates a two-dimensional graphical representation ofexemplary results of Example 4;

FIG. 12D illustrates a three-dimensional graphical representation ofexemplary results of Example 4;

FIG. 13A illustrates a lidar system used in example 5;

FIG. 13B illustrates a lidar system arranged over an experimental setupof example 5;

FIG. 13C illustrates a graphical representation of exemplary results ofExample 5;

FIG. 13D illustrates aspects of the experimental setup of example 5;

FIG. 13E illustrates aspects of the experimental setup of example 5;

FIG. 13F illustrates an experimental setup of example 5;

FIG. 13G illustrates a graphical representation of exemplary results ofexample 5;

FIG. 13H illustrates a graphical representation of exemplary results ofexample 5;

FIG. 13I illustrates a graphical representation of exemplary results ofexample 5;

FIG. 13J illustrates a graphical representation of exemplary results ofexample 5;

FIG. 14A illustrates an experimental setup of example 6;

FIG. 14B illustrates an experimental setup of example 6;

FIG. 14C illustrates a graphical representation of exemplary results ofexample 5;

FIG. 15A illustrates a graphical representation of exemplary results ofexample 6;

FIG. 15B illustrates a flight path according to example 6; and

FIG. 16 illustrates a picture of an office building measured with lidardepolarization ratio according to example 8.

In the appended figures, similar components and/or features may have thesame reference label. Further, various components of the same type maybe distinguished by following the reference label by a letter thatdistinguishes among the similar components. If only the first referencelabel is used in the specification, the description is applicable to anyone of the similar components having the same first reference labelirrespective of the second reference label.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

The present disclosure can provide a number of advantages depending onthe particular aspect, embodiment, and/or configuration. These and otheradvantages will be apparent from the disclosure.

The phrases “at least one,” “one or more,” and “and/or” are open-endedexpressions that are both conjunctive and disjunctive in operation. Forexample, each of the expressions “at least one of A, B and C,” “at leastone of A, B, or C,” “one or more of A, B, and C,” “one or more of A, B,or C” and “A, B, and/or C” means A alone, B alone, C alone, A and Btogether, A and C together, B and C together, or A, B and C together.

The term “a” or “an” entity refers to one or more of that entity. Assuch, the terms “a” (or “an”), “one or more” and “at least one” can beused interchangeably herein. It is also to be noted that the terms“comprising,” “including,” and “having” can be used interchangeably.

The term “automatic” and variations thereof, as used herein, refers toany process or operation done without material human input when theprocess or operation is performed. However, a process or operation canbe automatic, even though performance of the process or operation usesmaterial or immaterial human input, if the input is received beforeperformance of the process or operation. Human input is deemed to bematerial if such input influences how the process or operation will beperformed. Human input that consents to the performance of the processor operation is not deemed to be “material.”

The term “module,” as used herein, refers to any known or laterdeveloped hardware, software, firmware, artificial intelligence, fuzzylogic, or combination of hardware and software that is capable ofperforming the functionality associated with that element.

The terms “determine,” “calculate,” and “compute,” and variationsthereof, as used herein, are used interchangeably and include any typeof methodology, process, mathematical operation or technique.

It shall be understood that the term “means,” as used herein, shall begiven its broadest possible interpretation in accordance with 35 U.S.C.,Section 112(f). Accordingly, a claim incorporating the term “means”shall cover all structures, materials, or acts set forth herein, and allof the equivalents thereof. Further, the structures, materials or actsand the equivalents thereof shall include all those described in thesummary of the invention, brief description of the drawings, detaileddescription, abstract, and claims themselves.

As used herein, “comprising,” “including,” “containing,” “is,” “are,”“characterized by,” and grammatical equivalents thereof are inclusive oropen-ended terms that do not exclude additional unrecited elements ormethod steps unless explicitly stated otherwise.

The term “resolution” relates to a degree of discernibility ordistinguishability between two objects or events (e.g., light pulses).

The term “intrapulse” relates to events occurring within a single pulselength, also referred to as sub-pulse.

The term “interpulse” refers to the relation between two or more pulsesthat could be overlapping or separate in time.

The preceding is a simplified summary of the disclosure to provide anunderstanding of some aspects of the disclosure. This summary is neitheran extensive nor exhaustive overview of the disclosure and its variousaspects, embodiments, and/or configurations. It is intended neither toidentify key or critical elements of the disclosure nor to delineate thescope of the disclosure but to present selected concepts of thedisclosure in a simplified form as an introduction to the more detaileddescription presented below. As will be appreciated, other aspects,embodiments, and/or configurations of the disclosure are possible,utilizing, alone or in combination, one or more of the features setforth above or described in detail below.

In order to more fully appreciate the present disclosure and to provideadditional related features, the following references are incorporatedherein by reference in their entirety:

(1) U.S. Pat. No. 4,967,270 by Ulich, et al., which discloses animproved imaging light detection and ranging (lidar) system whichprovides variable time delay range gating across a selected image.Variable time delay range gating across an image is accomplished using aplurality of imaging cameras which are individually triggered afterpreselected delays to obtain multiple subimages. These multiplesubimages are then put together in a mosaic in a computer to provide acomplete image of a target plane using only one light pulse.

(2) U.S. Pat. No. 6,928,194 by Mai, et al., which discloses a system formosaicing multiple input images, captured by one or more remote sensors,into a seamless mosaic of an area of interest. Each set of input imagescaptured by the remote sensors within a capture interval areortho-rectified and mosaiced together into a composite image. Successivecomposite images, along a given flight path, are then mosaiced togetherto form a strip. Adjacent strips are then mosaiced together to form afinal image of the area of interest.

(3) U.S. Pat. No. 7,127,348 by Smitherman, et al., which discloses, avehicle based data collection and processing system which may be used tocollect various types of data from an aircraft in flight or from othermoving vehicles, such as an automobile, a satellite, a train, etc. Invarious embodiments the system may include: computer console units forcontrolling vehicle and system operations, global positioning systemscommunicatively connected to the one or more computer consoles, cameraarray assemblies for producing an image of a target viewed through anaperture communicatively connected to the one or more computer consoles,attitude measurement units communicatively connected to the one or morecomputer consoles and the one or more camera array assemblies, and amosaicing module housed within the one or more computer consoles forgathering raw data from the global positioning system, the attitudemeasurement unit, and the retinal camera array assembly, and processingthe raw data into orthorectified images.

(4) U.S. Pat. No. 7,580,127 by Mayor, et al., which discloses apolarization lidar system capable of remotely identifyingcharacteristics of atmospheric aerosol particles by transmitting apolarized beam of light and analyzing polarization characteristics ofreceived backscatter is disclosed. The transmitter features high pulseenergy to remotely identify aerosol particles with substantially onepulse. The transmitter employs a thin film plate polarizer and a Ramanwavelength shifter to achieve eye-safe, single-plane linearly polarizedenergy. The transmit beam and receiver field of view are coaxial. Thereceiver employs a telescope, a collimating lens, and a beam splitter.The beam splitter splits the received backscatter into a single-planepolarized beam whose polarization plane is parallel to the plane oftransmission and a single-plane polarized beam whose polarization planeis perpendicular to the plane of transmission. Each split beam isdirected through separate focusing lenses onto separate detectors. Thedetector signals are amplified and processed to remotely determineatmospheric aerosol particle characteristics.

(5) U.S. Pat. No. 7,725,258 by Smitherman, which discloses a vehiclebased data collection and processing system which may be used to collectvarious types of data from an aircraft in flight or from other movingvehicles, such as an automobile, a satellite, a train, etc. In variousembodiments the system may include: computer console units forcontrolling vehicle and system operations, global positioning systemscommunicatively connected to the one or more computer consoles, cameraarray assemblies for producing an image of a target viewed through anaperture communicatively connected to the one or more computer consoles,attitude measurement units communicatively connected to the one or morecomputer consoles and the one or more camera array assemblies, and amosaicing module housed within the one or more computer consoles forgathering raw data from the global positioning system, the altitudemeasurement unit, and the retinal camera array assembly, and processingthe raw data into orthorectified images.

(6) U.S. Pat. No. 7,893,957 by Peters, III, et al., which discloses acamera system having a compound array of imaging sensors disposed in aretinal configuration. The system preferably comprises a concavehousing. A first imaging sensor is centrally disposed along the housing.At least one secondary imaging sensor is disposed along the housing,adjacent to the first imaging sensor. The focal axis of the secondimaging sensor, in the preferred embodiment, intersects with the focalaxis of the first imaging sensor within an intersection area.

(7) U.S. Pat. No. 7,899,311 by Kearney, et al., which discloses a camerasystem having a rigid camera frame to receive a removable shutter andserve as a releasable attachment point for the camera lens system andimage receiving means. An adaptor and wedge have complementary contactpoints that allow constant alignment and orientation of the systemcomponents after removal and reattachment without recalibration orrealignment. The camera system may be used in a photographic imagingsystem such as an airborne reconnaissance system.

(8) U.S. Patent Application Publication No. 2006/0231771 by Lee, et al.,which discloses a system for detecting airborne agents. The system caninclude a laser source that provides laser pulses of at least twowavelengths, a transmitter that transmits the laser pulses, and acoupling mechanism configured to remotely couple the laser pulsesbetween the laser source and the transmitter. The system can include areceiver that receives both elastically backscattered signals fromairborne agents and fluorescence signals from the airborne agents. Thesystem can include a telescope that both transmits a collimated laserbeam of the laser pulse from the transmitter to a far field and receivesthe elastically backscattered signals and the fluorescence signals fromthe far field. The system can include a detection system having at leastone of a backscatter optical detector that detects the elasticallybackscattered signals and one or more fluorescence optical detectorsthat detect the fluorescence signals in selected spectral band(s) fromthe airborne agents.

(9) U.S. Patent Application Publication No. 2008/0137058 by Cesare,which discloses monostatic LIDARs use the same telescope to send thelaser beam in atmosphere and to collect the backscattered echo. Animportant element of monostatic LIDARs is the optical separator betweenthe emission and reception paths of the laser beam. By using a systemmade by a Faraday rotator in combination with two polarizing beamsplitters suitably oriented, it is possible to achieve this separationwith minimum losses with respect to prior systems using semi-reflectiveplates and/or polarizing beam splitters in conjunction with quarter-waveplates. The effectiveness of this system does not rely on themaintenance of the polarization status of the laser beam whenbackscattered by the atmosphere molecules and particles, or on thereduction of the received laser power relatively to the transmitted one.The system is simple, compact, and can work at several wavelengths ofthe laser source.

(10) U.S. Patent Application Publication No. 2010/0025589 by Olcott, etal., which discloses methods and systems for processing an analog signalthat is generated by a high energy photon detector in response to a highenergy photon interaction. A digital edge is generated representing thetime of the interaction along a first path, and the energy of theinteraction is encoded as a delay from the digital edge along a secondpath. The generated digital edge and the delay encode the time andenergy of the analog signal using pulse width modulation.

Embodiments herein presented are not exhaustive, and further embodimentsmay be now known or later derived by one skilled in the art.

One embodiment is directed towards a lidar system. A lidar system isconfigured to measure the distance or other properties of a targetsurface by illuminating the target surface with light. In oneembodiment, the lidar system of the present invention is configured tomeasure shallow depths of semi-transparent media. Media includes a firstsurface and a second surface and the body of the media in-between thefirst and second surfaces. In a vertical orientation, the first andsecond surfaces of the media may be the top and bottom surfaces,respectively. Light transmitted is partially scattered from andpartially refracted into the top surface of semi-transparent media. Thebottom surface may include a relatively opaque or polarization-alteringmedia or a second semi-transparent media. The top surface and the bodyof semi-transparent media include but are not limited to media such aswater or glass. The bottom surface that is relatively opaque orpolarization-altering includes but is not limited to media such as ice,sand, rock, wall, skin, hypodermis, cells, other anatomical regions,and/or combinations of the same.

In one embodiment, the lidar system includes a transmitter configured tooutput polarized light to a target, a receiver configured to collectscattered light from the target, and first and second detectors. Thefirst and second detectors are configured to receive at least arespective portion of the scattered light from the receiver. The firstand second detectors may be configured to detect the respective portionsof the scattered light and substantially the same time or with someoffset. The system also includes timing electronics coupled to each ofthe first and second detectors.

The transmitter used in the invention may include transmitters thatgenerate and transmit a light with a known polarization. The transmittermay include a laser and a polarizer in optical communication with thelaser. The transmitter may further include a half-wave plate in opticalcommunication with the laser. The laser may include at least one of apolarized laser, a pulsed laser, or a continuous wave (CW) laser. In onespecific example, a transmitter includes a Teem Photonics 35 mW laserwith a 450 ps pulse width with optics that generates and transmitslinearly polarized light with degree of polarization greater than 99.9%.

The receiver may include one or more receivers that can receivescattered light. The receiver may include a telescope. The receiver mayoptionally include one or more components for filtering processes (i.e.a spectral filter) and may further include a polarizing splitter. In onespecific example, a receiver includes an Orion Maksutov-Cassegraintelescope with a 90 mm aperture and a 1250 mm effective focal length.

The first and second detectors include detectors that can detect andcount photons in a light signal. The detectors may includephotomultiplier tubes that output a photon count signal. Other detectorsthat may operate in photon counting or analog mode include avalanchephotodiodes, charge coupled devices, or other photon detectors. In onespecific example, the detectors include Hamamatsu H7422PA-40photomultiplier tubes with a 2.5 ns pulse width.

Timing electronics includes electronics that can calculate a relativedistance based on an amount of time elapsed between light signals.Timing electronics may include a constant fraction discriminator (CFD)to discriminate an apex in a photon count signal, a time-to-digitalconverter (TDC), and a processor. In one specific example, timingelectronics include a SensL CFD with an 8 ns output pulse width and aSensL HRM Time TDC with a 27 ps bin width and a 190 ns dead time.

In another embodiment, a lidar system includes a light transmitterconfigured to transmit a light signal, a light receiver configured toreceive a scattered light signal, the scattered light signal includesfirst and second components, and a detector configured to resolve thefirst and second components of the scattered light signal.

A light signal includes electromagnetic radiation carrying information.Information includes distinguishing attributes of the light signal suchas the amplitude, frequency, phase, polarization, other attributes,and/or combinations of the same. The light signal may be coded bynatural (i.e., light containing a polarization signature of the targetscattered from a linearly polarized incident light) or artificial means(i.e., coding embedded by electronics when a light is generated). Inaddition, the polarization may be any type of polarization (e.g.,linearly, vertical, horizontal, and/or circular). A light signal may beany type of signal (e.g., pulsed or continuous wave (CW) laser, lamp,LED light, and/or other light sources or combinations of the same).Pulsed light signals may have demarcations in the null signal betweenthe pulses; CW light signals may have demarcations where the wavechanges modulation, phase, and/or other attributes. A light signal mayinclude component signals with varying attributes occupying at least aportion of substantially the same and/or indistinguishable time and/orspace as the light signal.

The scattered light signal includes the specular and diffuse lightscattered from the targeted medium. A scattered light signal includes adirected light signal changing direction as a result of the directedlight signal hitting a surface. The surface could include polarizationpreserving or polarization-altering surfaces. The scattered light signalmay have a different intensity, frequency, phase, polarization, otherattributes, and/or combinations of the same, due to the characteristicsof the directed light signal interacting with the surface. Further, whenthe directed light signal hits a polarization-altering surface, thescattered light signal may scatter significantly to variouspolarizations and/or directions. A reflected light signal includes thespecular light scattered from a target medium. A reflected light signalmay further refer to the action of optical components within atransmitter and receiver of an instrument that directs a light signalfrom one element to another.

A component signal of a light signal includes at least some uniformdistinguishing attribute such as amplitude, frequency, phase,polarization, other attributes, and/or combination of the same. In onespecific example, the co-planar polarization and the cross-planarpolarization of a light signal are two components of a light signal.

In yet another embodiment, a lidar system includes a source of polarizedlight and a light receiver, the light receiver configured to receivescattered light. The light receiver includes a polarizing beam splitter.The polarizing beam splitter splits the scattered light into across-planar polarization component and a co-planar polarizationcomponent. The lidar system further includes a first detector and asecond detector. The first detector is configured to receive thecross-planar polarized component. The second detector is configured toreceive the co-planar polarized component.

In still another embodiment, a method of measuring a relative distancebetween a first surface and a second surface with differing polarizationcharacteristics. The method includes the steps of generating polarizedlight, scattering at least some of the polarized light from the firstsurface and at least some of the polarized light from the secondsurface, receiving the scattered light, and splitting the scatteredlight into a first and second component. The first and second componenthave a relative difference in polarization. The method further includesthe steps of detecting the first and second component, determining anamount of time elapsed between the first and second component, andcalculating a relative distance between the first surface and the secondsurface based on the amount of time elapsed.

In another embodiment, a lidar system includes a laser light source, anoutput of the laser light source is laser light. The lidar systemfurther includes a polarizing beam splitter, the laser light is alignedwith the transmission axis of the polarizing beam splitter, and anoutput of the polarizing beam splitter is linearly polarized light. Thelidar system further includes a quarter-wave plate, the linearlypolarized light transmitted by the polarizing beam splitter is receivedat the quarter-wave plate. In a first mode of operation, the fast orslow axis of the quarter-wave plate is oriented 45-degrees to thelinearly polarized light output of the polarizing beam splitter,circularly polarized light is emitted by the quarter-wave plate. In asecond mode of operation, the quarter-wave plate is oriented such thatfast and slow axis of the quarter-wave plate are aligned with the linearpolarized light output of the polarizing beam splitter, linearlypolarized light is emitted by the quarter-wave plate. The lidar systemfurther includes a detector. Light scattered by the polarizing beamsplitter is received at the detector.

In yet another embodiment, a lidar system includes a source of light anda variable wave plate. In a first mode of operation, the variable waveplate is configured to output light polarized in a first direction, andin a second mode of operation, the variable wave plate is configured tooutput light polarized in a second direction relatively different thanthe first direction. The lidar system further includes a detector. Thedetector is configured to receive scattered light of the polarizedlight.

In still another embodiment, a method of measuring a relative distancebetween surfaces includes measuring a relative distance to apolarization preserving surface, which includes generating linearlypolarized light at a first time, the generated light is verticallypolarized, circularly polarizing the vertically polarized light in afirst direction, and scattering at least some of the light circularlypolarized in a first direction from the polarization preserving surface.The scattered light is circularly polarized in a second direction afterbeing scattered by the polarization preserving surface. Measuring therelative distance between surfaces further includes linearly polarizingthe scattered light, and passing the linearly polarized light to adetector. The light is received at the detector at a second time.Measuring the relative distance between surfaces further includesdetermining an amount of time elapsed between the first time and thesecond time to obtain a first time difference, and measuring a relativedistance to a polarization-altering surface. Measuring a relativedistance to the polarization-altering surface includes generatinglinearly polarized light at a third time, the generated light isvertically polarized, passing at least a first portion of the verticallypolarized light through the polarization preserving surface to thepolarization-altering surface, a second portion of the verticallypolarized light is scattered by the polarization preserving surface asvertically polarized light, scattering the linearly polarized lightpassed through the polarization preserving surface from thepolarization-altering surface, the linearly polarized light is alteredafter being scattered by the polarization-altering surface, passing ahorizontally polarized component of the polarization-altered scatteredlight to a detector, the light is received at the detector at a fourthtime, light scattered by the polarization preserving surface is notpassed to the detector, determining an amount of time elapsed betweenthe third time and the fourth time to obtain a second time difference,using the first and second time differences, and calculating a relativedistance between the polarization preserving surface and thepolarization-altering surface.

In another embodiment, a method of measuring relative distance between afirst surface and a second surface with different polarizationcharacteristics includes generating light, scattering the lightrespectively from the first surface and the second surface, receivingeach of the scattered light from the first surface and the secondsurface, detecting the scattered light, and determining an amount oftime elapsed between the light scattered from the first surface and thelight scattered from the second surface.

In one embodiment, the lidar system can be configured for using in anunderwater vehicle as part of or an input for a navigation system ormethod.

In one embodiment, the lidar system is utilized in a vehicle to providereal-time navigation. The navigation may include autonomous navigationfor a vehicle, used as in an additional input to aid with mannednavigation, and/or an additional input for autonomous navigation. Thevehicle can include any type of vehicle, e.g., aircraft manned orunmanned, automobile, autonomous automobile, underwater vehicle in anunderwater environment, e.g., ocean, lake, river, and the like.

The navigation system can include many different types of components ofa lidar system. The navigation system may be part of or independent of anavigation system of an autonomous underwater vehicle (AUV) which is arobot that can operate in underwater and unstructured environmentswithout continuous human guidance, submarine, an unmanned underwatervehicle (UUV) which is a classification vehicle that includesnon-autonomous remotely operated underwater vehicles (ROVs), e.g.,controlled and powered from the surface by an operator/pilot via anumbilical or using remote control and other vehicles, e.g., a submarineor device to aid an underwater scuba diver.

In one embodiment, the lidar is configured to provide navigation orobject detection to assist with navigation and/or mapping of underwaterfeatures or objects or add in any of the foregoing, e.g., ocean floors,river beds, obstructions, and the like. It can also be used for otherpurposes, e.g., navigation for spacecraft, location of lost or sunkentreasure, boats, or other objects, underwater mine detection, and thelike.

In one embodiment, the lidar system includes a pulse of polarized lightthat is emitted through a semi-transparent media into aqueousenvironment. There is an aqueous/semi-transparent orunderwater/semi-transparent media interface.

In one embodiment, the lidar system is configured to utilize multiplewavelengths in one system or multiple wavelengths in multiple systems,e.g., in the same pulse or different pulses of energy. In a preferredembodiment, the wavelength may be in a range from about 250 nm to about10 μm. The selection of specific wavelength or multiple wavelengths isbased on the application or the surfaces to be measured.

In one embodiment, the lidar system is integrated with or into anavigation system, the navigation system includes one or more of aglobal positioning system (GPS), an inertial measurement unit (IMU), aradar unit, a laser rangefinder unit, a camera unit, a sonar unit, apower unit, a data storage unit, e.g., networked attached storage unit,a hard disk, a solid state drive (SSD) and the like. The navigationsystem is configured to communicate with other internal or externalcomponents. The lidar system is configured to provide more accuratenavigation along with other benefits described herein.

The GPS may be any sensor configured to estimate a geographic locationof the vehicle. To this end, GPS may include a transceiver operable toprovide information regarding the position of the vehicle with respectto the earth. The IMU unit can include any combination of sensors (e.g.,accelerometers and gyroscopes) configured to sense position andorientation changes of the vehicle based on inertial acceleration.

The radar system is configured to use radio signals to sense objectswithin the local environment of the vehicle. The sonar system isconfigured to use soundwaves to sense objects within the water. Thelidar, radar, sonar, and/or GPS may additionally be configured to sensethe speed and/or heading of the objects.

In one embodiment, the lidar system either independent or as a componentof a navigation system is configured to be used with or in an underwatervehicle described with reference to U.S. Pat. No. 9,090,320, which ishereby incorporated by reference as if fully set forth herein.

In one embodiment, the lidar system may be any sensor configured tosense objects in the environment in which the underwater vehicle islocated using one or more pulse of polarized light. Depending upon theembodiment, the lidar system can include one or more laser sources, alaser scanner, and one or more detectors, among other system components.The lidar system can be configured to operate in a coherent (e.g., usingheterodyne detection) or an incoherent detection mode.

In one embodiment, the interface at the semi-transparent media andunderwater is configured to remove or reduce any obscurance. In apreferred embodiment, the vehicle is configured with an object to createa laminar flow of water at or near the interface (semi-transparentmedia/liquid) and/or with a material to aid in creating laminar flow ofliquid at or near the interface, e.g., coating.

In one embodiment, the navigation system includes as a component a lidarsystem configured for three-dimensional mapper of an object, e.g.,underwater mapping of an object or surface, planetary or asteroidsurfaces and the like. The system is configured to provide resolution inthe range from about 60 meters or more with a few [cm] resolution. In apreferred embodiment, the range is from 30 m with a few [cm] resolution.

In one embodiment, the lidar system is configured with an objectidentification module, e.g., allowing the system to predict a type of anobject based on characterization of rough or smooth surface properties.

In one embodiment, the lidar system is configured with anauto-correction module that provides different adjustments to the systemas a function of index of refraction due to one or more of temperature,salinity, depth, and pressure.

In one embodiment, the lidar system comprises a duty cycle power modulethat allows the system to operate in different power modes, off, low,medium, and high to preserve battery and increase operation time. Thisduty cycle power module is configured to regulate power to all systemsubcomponents. For example, the duty cycle power module can beconfigured to utilize different processing times for the lidar systemand operation times of the lidar system, e.g., operations may be set inrange from about every 5 minutes to about every day, week, month, ormore. This is especially useful for remote or satellite lidar system inorder to conserve energy or operate when solar energy units are at theirmaximum. In one embodiment, the vehicle includes a transmitterconfigured to output a pulse of polarized light to a target, a receiverconfigured to collect scattered light from the target, one or moredetectors configured to receive at least a respective portion of thescattered light from the receiver, and timing electronics coupled toeach of the one or more detectors.

In one embodiment, the vehicle includes a light transmitter configuredto transmit a light signal, a light receiver configured to receive ascattered light signal, the scattered light signal includes a firstcomponent and a second component, and a detector configured to resolvethe first and second components of the scattered light signal to asub-pulse width resolution.

In one embodiment, the vehicle includes a lidar system having a sourceof polarized light and a light receiver, the light receiver configuredto receive scattered light. The light receiver includes a polarizingbeam splitter, the polarizing beam splitter splits the scattered lightinto a cross-planar polarization component and a co-planar polarizationcomponent. The lidar system further includes a first detector, the firstdetector is configured to receive the cross-planar polarized component,and a second detector, the second detector is configured to receive theco-planar polarized component. Reference will now be made in detail toan embodiment of the present invention, examples of which areillustrated in the accompanying drawings.

FIG. 1 illustrates a lidar system 104 in accordance with embodiments ofthe present invention, in an exemplary operating environment 100. Thelidar system 104 generates transmitted light 108 that is directedtowards a target 112. The target 112 may comprise a body of water 116having a top surface 120 and a floor 124. In a first mode of operation,light 128 scattered from the surface 120 of the target 112 is receivedby the lidar system 104. The time elapsed between the generation of apulse of light 108 scattered from the surface 120 of the target 112 andreturned to the lidar system 104 as a scattered signal 128 is used todetermine a relative distance between the surface 120 of the target 112and the lidar system 104. In a second mode of operation, the timeelapsed between the generation of a pulse of transmitted light 108 and asignal 132 scattered from the floor 124 of the target 112 is used todetermine the relative distance between the lidar system 104 and thefloor 124 of the target 112. By taking the difference between thedistance to the surface 120 and the distance to the floor 124, therelative distance between the surface 120 and the floor 124 can bedetermined. Accordingly, the relative depth of the water 116 can bedetermined. In the example of FIG. 1 , the lidar system 104 isassociated with a platform 136 comprising an airplane. However, a lidarsystem 104 in accordance with embodiments of the present invention maybe associated with different platforms 136. Examples of suitableplatforms 136, in addition to an airplane, include satellites, unmannedaerial vehicles, helicopters, balloons, boats, or other platforms. Inaddition, a lidar system 104 in accordance with embodiments of thepresent invention is not limited to shallow water bathymetry. Forexample, the lidar system 104 can be used for bottom surface mapping, orfor determining the distance between any polarization preserving surfacethat is at least partially transmissive of light 108, and apolarization-altering surface behind the polarization preservingsurface, particularly in the instance where the separation distancewould be otherwise unresolvable due to system bandwidth limitationsassociated with laser and/or detector pulse widths. Thepolarization-altering surface includes but is not limited to media suchas ice, sand, rock, wall, skin, hypodermis, cell, other anatomicalregions, and combinations of the same.

FIG. 2A and FIG. 2B illustrate detection of scattered light pulse overtime for a system with pulse width resolution.

FIG. 2A depicts an exemplary shallow water environment 280 with shallowwater body 203 having water surface 201 and water floor 202. The y-axisrepresents the distance of a vertical cross-section of shallow waterenvironment 280. The x-axis represents time. The level of water surface201 is at distance h=0; the level of water floor 202 is at distance h.Water body 203 with a water medium has a refractive index of n=1.33. Airmedium above water surface 201 has a refractive index of n=1.

Transmitted (Tx) pulse 210 is a light pulse having a length ci. At timeto, Tx pulse 210 is generated by a lidar system such as lidar system 104or by other light sources. In environment 280, Tx pulse 210 is beingtransmitted substantially normal to water surface 201. However, Tx pulse210 may be transmitted at other angles as long as Tx pulse 210 can be atleast partially scattered from and partially refracted through watersurface 201.

At time t_(surface), Tx pulse 210 arrives at water surface 201. Asstated, Tx pulse 210 will be partially scattered off water surface 201,the scattered light pulse being received (Rx) pulse 220, and partiallyrefracted through water surface 201 into water body 203, the refractedlight pulse being refracted pulse 240. Thus, at time τ/2, half of Txpulse 210 has been scattered as Rx pulse 220 with length cτ/2, and halfof Tx pulse 210 has been refracted as refracted pulse 240 with length0.376cτ (due to the refraction index in water body 203). At time τ, Txpulse 210 has been fully either scattered as Rx pulse 220 or refractedas refracted pulse 240.

At time t_(floor), refracted pulse 240 reaches water floor 202 and willbe at least partially scattered as Rx pulse 230. Rx pulse 230, likerefracted pulse 240, will have a comparatively shortened length whentraveling in water body 203 because of the refractive index of waterbody 203 (n=1.33) as opposed to air (n=1). Rx pulse 230 will lengthen tolength ci when it exits the water surface 201.

Thus, when Rx pulse 230 exits water surface 201, the time differencebetween t_(floor) and t_(surface) can be derived from the timedifference between Rx pulse 220 and Rx pulse 230. Further, therelationship between the time difference of t_(floor) and t_(surface)and the physical distance between water surface 201 (h=0) and waterfloor 202 (h) is given by

$\begin{matrix}{h = \frac{c\Delta t}{2n}} & (1)\end{matrix}$

Therefore, the depth of water body 203 can be determined.

FIG. 2B depicts timings diagrams for Rx pulses 220 and 230 for specificwater depth scenarios. When the distance between water surface 201 andwater floor 202 (h) is greater than a minimum depth of water (hmin) forwhich half of Rx pulse 220 has scattered from water surface 201(h>hmin), a discernible gap exists between Rx pulses 220 and 230, andtiming difference between Rx pulses 220 and 230 (Δt) is tfloor-tsurface.When the distance between water surface 201 and water floor 202 (h) isequal to hmin, Rx pulse 230 comes directly after Rx 220 with nodiscernible gap and no overlapped portions between Rx pulses 220 and230. When the distance between water surface 201 and water floor 202 (h)is less than hmin (h<hmin), Rx pulse 230 comes before the entire portionof Rx pulse 220 has progressed, creating an ambiguous interpulse overlap225.

Therefore, there is a limitation to the detection method as described inFIG. 2A and FIG. 2B. This limitation is that the scattered pulses Rxpulse 220 and Rx pulse 230 must be substantially separable. That is, Rxpulse 230 must not start to exit water surface 201 before Rx pulse 220has been completely scattered from water floor 202. Effectively, thisrequirement requires a minimum depth of water (h_(min)) for which Rxpulse 230 cannot scatter from water floor 202 before half of Rx pulse220 has scattered from water surface 201 at time τ/2. In thisembodiment, h_(min) is 0.376cτ (due to the refraction index in waterbody 203 as discussed previously) and depends on the length of Tx pulse210.

When h<h_(min), the two scattered pulses, Rx pulse 220 and 230, have anambiguous interpulse overlap 225 that is not separable for resolving thetime difference between t_(floor) and t_(surface) from Rx pulses 220 and230. In practice, h_(min) is limited by equipment limitations forgenerating and detecting light pulses with minimal length i.

FIG. 2C and FIG. 2D illustrate detection of scattered light pulse overtime for a system with intrapulse or sub-pulse width resolution inaccordance with an embodiment of the present invention.

Referring to FIG. 2C, an exemplary shallow water environment is depictedas reference number 290. The environment 290 includes a shallow waterbody 206 having water surface 204 and water floor 205. The y-axisrepresents the distance of a vertical cross-section of shallow waterenvironment 290. The x-axis represents time. The level of water surface204 is at distance h=0, and the level of water floor is at distance h.The depth of water body 206 is at distance h, which is less than theminimum depth of water (h_(min)) for Tx pulse 230. Therefore, an Rxpulse 240 that is scattered from water surface 204 and an Rx pulse 250that is scattered from water floor 205 includes an ambiguous interpulseoverlap portion 245. Ambiguous interpulse overlap 245 is createdsimilarly to ambiguous interpulse overlap 225 as described with respectto FIG. 2A and FIG. 2B.

In this embodiment, Tx pulse 230 is a light pulse having a knownpolarization. For example, Tx pulse 230 is polarized in the cross-planardirection to the propagation vector of Tx pulse 230, which is normal towater surface 204 when in direct nadir viewing. Tx pulse 230 can bepolarized by a lidar system, such as lidar system 104, or by otherpolarizing light sources as known in the art. As Tx pulse 230 arrives atwater surface 204 at time t_(surface), Tx pulse 230 is partiallyscattered by water surface 204 as Rx pulse 240. Since water surface 204is a polarization preserving surface, Rx pulse 240 keeps substantiallythe same polarization as Tx pulse 230. Tx pulse 230 is also partiallyrefracted into water body 206. The refracted portion of Tx pulse 230 isscattered by water floor 205 at time t_(floor). Water floor 205 is apolarization-altering surface and creates polarization scattering in thescattered light. Therefore, Rx pulse 250 will have a differentpolarization from Tx pulse 230 and Rx pulse 240 when scattered fromwater floor 205.

FIG. 2D depicts timings diagrams for Rx pulses 240 and 250 for waterdepth of h<h_(min). Scattered pulses Rx pulse 240 and Rx pulse 250 willhave an overlap 245 because of h<h_(min). Referring to views 1 and 2, inthis embodiment, the ambiguous interpulse overlap 245 can be removed andRx pulses 240 and 250 can be separated as two distinct signals becauseRx pulse 240 has only the cross-planar polarization being scattered fromwater surface 204, and Rx pulse 250 has a range of altered polarizationsdue to the backscattering from being scattered from water floor 205.This separation may be achieved by various mechanical (i.e.,mechanically movable mirrors), optical (i.e., prisms or splittingpolarizers), electronic means (i.e., photon counting detectors), and/orcombination of the same.

While FIGS. 2A-2D were discussed with respect to one transmittedpolarized light pulse according to an embodiment of the invention, otherconfigurations can be used. For example, instead of pulsed light,continuous wave (CW) laser can also be used (i.e., where gaps betweeneach “pulse” can be similarly obtained by modification of phase inducedby scattering). Further, two or more transmitted light pulses withdifferent polarizations can also be used in place of or in complement tothe one cross-planar polarized light. For example, according to oneembodiment of the invention as discussed with reference to FIG. 5 , twolight pulses can be transmitted each having a different polarizationsuch that, after filtering, one pulse will gather signal only from thepolarization preserving surface and one pulse will gather signal onlyfrom the polarization-altering surface. In this configuration, only onedetector is required to count both light pulses. Still further,polarizations that are in alignment with the transmitted pulse (i.e.,co-planar and cross-planar polarization) are preferred but are notrequired. Other polarization angles can be used and may be better suitedfor other applications (i.e., surfaces positioned at an angle orsurfaces made up of other materials such as ice).

FIG. 3 illustrates a lidar system according to an embodiment of theinvention. Referring to FIG. 3 , lidar system is generally depicted asreference number 300. The lidar system 300 includes a light transmitter310, light receiver 330, and timing electronics 340. The lighttransmitter 310 is configured to generate and output at least one lightsignal (e.g., pulsed or continuous wave (CW) laser). In a preferredembodiment, the outputted light signal has a known polarization. Target320 is a shallow water body or any other type of body with a respectiverelatively polarization preserving and semi-transparent surface (firstsurface) and a relatively polarization-altering (e.g., opaque and/ordepolarization) surface (second surface). The outputted light signalfrom light transmitter 310 is configured to scatter from both the firstand second surfaces. Light receiver 330 is configured to receive thescattered light signals from target 320 and separate the scattered lightsignals into their respective components. Timing electronics 340 iselectrically coupled to light receiver 330 and is configured tocalculate a relative distance based on an amount of time elapsed betweenlight signals.

FIG. 4 illustrates components of a lidar system according to anembodiment of the invention.

Referring to FIG. 4 , the lidar system 400 includes light transmitter410, light receiver 430, and timing electronics 440. In this embodiment,the light transmitter 410 includes laser 412, beam expander 413,half-wave plate 414, polarizer 415, and prisms 416. Laser 412 acts as alight source for lidar system 400 and is configured to emit a focusedlight as the basis of the transmitted light signal. Laser 412 can be apulsed laser, continuous wave (CW) laser, polarized laser, or othertypes of lasers. In other embodiments, laser 412 can generically includeother light sources as known in the art (i.e., lamp or LED light). Inone embodiment, a 450 ps pulsed laser is used as laser 412. Beamexpander 413, half-wave plate 414, polarizer 415, and prisms 416 areoptional and are configured to focus and align the transmitted lightsignal towards target 420. In this embodiment, the chain of beamexpander 413, half-wave plate 414, polarizer 415, and prisms 416 areeach aggregated and aligned to the optical path of the transmitted lightsignal. Beam expander 413 is configured to expand the transmitted lightsignal for tight spot targets. Half-wave plate 414 may be mechanicallyor electrically (i.e., using a liquid crystal variable retarder)operable to control the retardance of the focused light signal along theoptical path. Polarizer 415 is configured to polarize the light signalwith a known polarization. A polarizing laser may also be used as laser412 for a known polarization. Prisms 416 are configured to coaxiallydirect and focus the transmitted light signal to target 420 as known inthe art.

In operation, trigger 411 may be electrically coupled to laser 412 orother components of light transmitter 410 to start the transmission ofthe light signal. In other embodiments, light transmitter 410 mayoperate continuously without trigger 411. Light signal is transmittedfrom light transmitter 410 to target 420. Target 420 includes at least afirst surface and a second surface as described herein. The transmittedlight signal is partially scattered from the first surface as a firstscattered light signal and partially refracted into the target. Therefracted light is scattered from the second surface as a secondscattered light.

The first scattered light signal has substantially the same polarizationas the transmitted light signal while the second scattered light signalwill have a different polarization due to the scattering from the secondsurface. The first and second scattered light signals may have anoverlapped interpulse portion forming one combined scattered lightsignal.

Light receiver 430 includes telescope 431, field stop 432, spectralfilter 433, splitting polarizer 434, first detector 435, and seconddetector 436. Each of these components are aggregated and aligned to anoptical path of the scattered light signal. Telescope 431 acts tocollect the scattered light signal. Field stop 432 and spectral filter433 are optional components. Field stop 432 acts to limit the field ofview of light receiver 430 where the scattered light signal would begathered. Spectral filter 433 acts to further filter the received lightto the light spectrum of interest (i.e., limiting the spectrum to theexpected frequency of the scattered light signals).

Splitting polarizer 434 acts to separate the received scattered lightsignal according to the polarization. In this embodiment, thepolarization splitter 434 is aligned with the optical path of thescattered light signal. As scattered light signal reaches polarizationsplitter 434, the cross-planar polarized component of the signalsubstantially passes through polarization splitter 434 while theco-planar polarized component of the signal substantially reflects. Theangle of reflection is a function of the type of polarizer used (i.e.90° angle or 62° angle for a Glan Taylor polarizer). Here, the firstscattered light signal scattered from the water surface containingcross-planar polarized light is substantially reflected (i.e., at a 90°angle) while at least the co-planar polarization component of the secondscattered light signal scattered from the water floor containingdepolarized light is substantially transmitted. Other orientations arealso possible depending on the polarization methodology used on thetransmitted light signal and the type of polarizer used for polarizationsplitter 434.

Detector 435 is positioned at a 180° optical path from the reflectedlight signal and configured to detect the cross-planar polarizationcomponent of scattered light signal.

Detector 436 is positioned at the optical path of the reflected lightsignal (i.e., 90°) and is configured to detect the co-planar componentof the scattered light signal. As such detector 436 is configured todetect the first scattered signal from the water surface while detector435 is configured to detect the second scattered signal from the waterfloor.

Detectors 435 and 436 may be photomultiplier tubes and are configured tocount the volume of photons in each signal within a certain timeinterval representing the strength of the signal and output a photoncount signal. In one embodiment, detectors 435 and 436 have 2.5 nsresolution. Moreover, the detectors 435 and 436 can be configured tosubstantially simultaneously detect separated scattered signals from thepolarizing beam splitter 434.

It is noted that polarizing beam splitter 434 can be positioned at avariety of angles to split the scattered light signal at other angles.Detectors 435 and 436 can be positioned at other configurations toreceive such split components of the scattered light signal.

The timing electronics 440 may include a constant fraction discriminator(CFD) 441, time-to-digital converter (TDC) 442, and processor 443.Processor 443 is coupled to CFD 441 and TDC 442 through a control linefor control and feedback of these components. CFD 441 is coupled todetectors 435 and 436 through a signal conditioning line and isconfigured to output an apex of the photon count signal at certainintervals representing the time at which the signal has meaningfullyarrived. In one embodiment of the invention, CFD 441 has an 8 nsresolution. TDC 442 is coupled to CFD 441 and is configured to convertthe time signal output by CFD 441 into a digital signal. In oneembodiment of the invention, TDC 442 has a resolution of 27 ps.

Processor 443 is coupled to TDC 442 and is configured to take thedigitized timing signal and determine the time of arrival of eachcomponent (co-planar and cross-planar polarized signals in oneembodiment) and calculate the difference in the time of arrival of thetwo signals. In this embodiment, the processor 443 is further configuredto transform the time difference into the depth between the polarizationpreserving and polarization-altering surfaces depending on thecalibration of the lidar system 400 and the refraction index of shallowwater body or other types of bodies in question. Further description ofthis calculation will be described with respect to FIG. 5 .

Further, an initial calibration to lidar system 400 may be neededbecause the light paths to detectors 435 and 436 may not be the sameafter the scattered light is separated by polarizing splitting 434.According to one embodiment, this calibration can be accomplished byusing a scattered signal from a surface that is depolarizing and notinga difference in the assessment of distance to that surface between thedetectors 435 and 436. The difference in the assessment of distance islikely due to the slightly different optical paths between each ofdetectors 435 and 436 and polarizing splitter 434. In one embodiment,this calibration can be performed once and saved for adjustment byprocessor 443. The correction and adjustment can be applied tosubsequent depth data by processor 443.

FIG. 5 illustrates components of a lidar system in accordance withembodiments of the present invention.

Referring to FIG. 5 , the lidar system 504 includes an optical bench orother structure, to which other components may be directly or indirectlyconnected. These components include a light source (e.g., pulsed or CWlaser) or laser 508. The laser 508 may be operated to generate linearlypolarized light 512 that is transmitted along an optical axis 516. As anexample, the linearly polarized light 512 may have a wavelength of 532nm. A half-wave plate 520 can be included along the optical axis 516.The half-wave plate 520 may be rotated about the optical axis 516, atleast during a calibration stage, to control the orientation of thelinearly polarized light 512 about the optical axis 516.

A polarizing beam splitter (PBS) 524, such as a polarizing beam splittercube, is located along the optical axis 516. Where required to maximizetransmission, the linearly polarized light 512 can be rotated about theoptical axis 516 by the half-wave plate 520, such that the light 512 isaligned with the transmission axis of the PBS 524, enabling the maximumamount of linearly polarized light 512 to pass through the PBS 524.

A quarter-wave plate 528 is located along the optical axis 516 such thatthe linearly polarized light 512 transmitted by the PBS 524 is passedthrough the quarter-wave plate 528. In addition, the quarter-wave plate528 is free to rotate about the optical axis 516. As will be describedin greater detail elsewhere herein, the quarter-wave plate 528 may berotated between a first orientation, in which the quarter-wave plate 528acts to circularly polarize the light 512 received from the polarizingbeam splitter, and a second orientation, in which the quarter-wave plate528 is aligned so as to maintain the linear polarization state of thelight 512 received from the polarizing beam splitter 524. As analternative, an electronically controlled variable wave plate may beused in place of the quarter-wave plate 528.

A detector 536 is located to receive light scattered from a target 513back through the quarter-wave plate 528, and that is in turn reflectedby the PBS 524. The detector 536 may, for example, comprise aphotomultiplier tube, an avalanche photodiode, a charge coupled device,or other light detector.

In a first mode of operation, the fast axis of the quarter-wave plate528 is oriented 450 to the linear polarization output of the PBS 524. Inthis orientation, the quarter-wave plate 528 retards the linear slowpolarization component of the light 512 transmitted from the laser 508through the PBS 524 relative to the fast polarization component of thatlight 512 by 90°, resulting in the emission of light towards the target513 that is circularly polarized in a first direction. The surface of atarget 513 comprising a body of water is polarization preserving.Accordingly, the circularly polarized light is scattered back to thelidar 504 in the opposite circular polarization state. For example,where the light 508 transmitted to the target 513 is left handcircularly polarized, the light scattered from the surface of the target513 will be right hand circularly polarized. The light scattered back tothe lidar system 504 is retarded again by the quarter-wave plate 528.The result is linearly polarized light that is rotated about the opticalaxis 516 by 90° as compared to the light 512 that originally exited thePBS 524. For example, where the light 512 exiting the PBS 524 wasvertically polarized, the light scattered by the polarization preservingsurface will be horizontally polarized after again passing through thequarter-wave plate 528. Accordingly, the scattered light is aligned withthe reflection axis of the PBS 524, and is directed by the PBS 524 tothe detector 536. Accordingly, a light pulse comprising scattering fromthe surface of the target 513 is delivered to the detector 536.

In the second mode of operation, the quarter-wave plate 528 is rotatedsuch that the fast and slow axes are aligned with the transmission planeof the PBS 524. The vertical polarization components of the transmittedlight are thus retarded equally by the quarter-wave plate 528. As aresult, linearly (e.g., vertically) polarized light is transmitted tothe target 513. Because the surface and the water column of a target 513comprising a body of water are polarization preserving, the linearpolarization state of the incident light is preserved. When scatteredback to the lidar system 504, this linearly polarized light is passedthrough the quarter-wave plate 528 unmodified, as linearly (e.g.,vertically) polarized light that is then transmitted by the PBS 524 backtoward the laser 508. Accordingly, the light scattered by the surface orthe water column of the target 513 is not delivered to the detector 536.However, the floor of the target 513 alters the polarization of thetransmitted linearly polarized light upon scattering. Thispolarization-altering effect is due to attributes such as the roughtopography of a typical floor of a body of water. Thepolarization-altered light scattered back to the lidar system 504 by thefloor of the target 513 is not altered by the quarter-wave plate 528.Accordingly, half of the light scattered from the floor of the target513 is reflected by the PBS 524 to the detector 536 for detection. Theother half of the light is transmitted through the PBS 524 back towardthe laser 508. As a result, in this second mode of operation, the lidarsystem 504 only passes light scattered from the floor of the target 513to the detector 536. This facilitates the detection of the floor, evenin connection with targets 513 comprising shallow (e.g., less than 10cm) bodies of water, since the earlier return from the surface andcolumn of the body of water is passed through the PBS 524 back towardthe laser 508, and therefore is not directed to the detector 536.

The scattering surface that is measured in shallow water bathymetryusing a lidar system 504 in accordance with embodiments of the presentinvention is dictated by the orientation of the rotating quarter-waveplate 528 and subsequent modulation of transmitted and receivedpolarization states. An analytical description of the technique beginsby defining the associated Stokes vector of the transmitted linearlypolarized laser pulse 512, S_(Tx), not limited to but for the purposesof demonstration here is oriented to an angle θ of π/4 radians about theoptical axis out of the laser transmission face.

$\begin{matrix}{S_{Tx} = \begin{bmatrix}1 \\0 \\1 \\0\end{bmatrix}} & (2)\end{matrix}$

The half-wave plate 520 used to rotate the linearly polarized laser 508light 512 into alignment with the transmission axis of the PBS 524 isexpressed as a variable wave plate with phase shift γ of π radians,oriented to rotate the linearly polarized laser 508 light 512 to thetransmission axis of the PBS 524. For the manifestation of the techniquedescribed here, the half-wave plate 520 is oriented to an angle θ of π/8radians about the optical axis. The resulting Mueller matrix for thevariable wave plate is defined as

$\begin{matrix}{{VWP} = {{\begin{bmatrix}1 & 0 & 0 & 0 \\0 & {\cos\left( {2\theta} \right)} & {- {\sin\left( {2\theta} \right)}} & 0 \\0 & {\sin\left( {2\theta} \right)} & {\cos\left( {2\theta} \right)} & 0 \\0 & 0 & 0 & 1\end{bmatrix}\begin{bmatrix}1 & 0 & 0 & 0 \\0 & 1 & 0 & 0 \\0 & 0 & {\cos(\gamma)} & {\sin(\gamma)} \\0 & 0 & {- {\sin(\gamma)}} & {\cos(\gamma)}\end{bmatrix}}{\begin{bmatrix}1 & 0 & 0 & 0 \\0 & {\cos\left( {{- 2}\theta} \right)} & {- {\sin\left( {{- 2}\theta} \right)}} & 0 \\0 & {\sin\left( {{- 2}\theta} \right)} & {\cos\left( {{- 2}\theta} \right)} & 0 \\0 & 0 & 0 & 1\end{bmatrix}}}} & (3)\end{matrix}$

The PBS 524 is modeled as a polarizer oriented to an angle θ of 0radians for transmission along the vertical axis in the instrumenttransmitter, and oriented to θ of π/2 radians for horizontaltransmission in the receiver.

$\begin{matrix}{{Pol} = {{\begin{bmatrix}1 & 0 & 0 & 0 \\0 & {\cos\left( {2\theta} \right)} & {- {\sin\left( {2\theta} \right)}} & 0 \\0 & {\sin\left( {2\theta} \right)} & {\cos\left( {2\theta} \right)} & 0 \\0 & 0 & 0 & 1\end{bmatrix}\begin{bmatrix}0.5 & 0.5 & 0 & 0 \\0.5 & 0.5 & 0 & 0 \\0 & 0 & 0 & 0 \\0 & 0 & 0 & 0\end{bmatrix}}{\begin{bmatrix}1 & 0 & 0 & 0 \\0 & {\cos\left( {{- 2}\theta} \right)} & {- {\sin\left( {{- 2}\theta} \right)}} & 0 \\0 & {\sin\left( {{- 2}\theta} \right)} & {\cos\left( {{- 2}\theta} \right)} & 0 \\0 & 0 & 0 & 1\end{bmatrix}}}} & (4)\end{matrix}$

During acquisition of bathymetric measurements, the quarter-wave plate528 is initially oriented to θ of π/4 radians for transmission ofcircularly polarized light towards the target 513 and then rotated to θof 0 radians for transmission of linear polarization. The quarter-waveplate 528 is expressed in terms of the variable wave plate Muellermatrix of (3), with phase shift γ of π/2 radians. As experienced alongthe return path of scattered signals, the quarter-wave plate 528 isexpressed with orientation θ of −π/4 radians for reception of circularlypolarized light and θ of 0 radians for reception of polarization-alteredsignals.

An example, normalized Mueller matrix that describes the scatterproduced by the target 513 water body incorporates a d term which rangesfrom 0 to 1 and describes the target's ability to alter the polarizationstate (depolarize) of the incident laser pulse polarization. In thissense, a d value of 0 corresponds to a polarization preserving targetsuch as the water surface, while a d value of 1 defines a completelydepolarizing target such as a rough floor topography.

$\begin{matrix}{{MDep}_{d} = \begin{bmatrix}1 & 0 & 0 & 0 \\0 & {1 - d} & 0 & 0 \\0 & 0 & {d - 1} & 0 \\0 & 0 & 0 & {{2d} - 1}\end{bmatrix}} & (5)\end{matrix}$

Combining the transmitted Stokes vector in (2) with the appropriateMueller matrices described in (3)-(5) produces the received Stokesvector

S _(Rx) =[VWP _(Tx) ^(λ/2)·Pol_(Tx) ·VWP _(Tx) ^(λ/4) ·MDep_(d) ·VWP_(Rx) ^(λ/4)·Pol_(Rx) ]S _(Tx)  (6)

the intensity of which, as measured by the detector 536, is defined as

I _(Rx)[1 0 0 0]S _(Rx)  (7)

FIG. 8 illustrates components of a lidar system according to anembodiment of the invention.

In an embodiment, the sub-pulse width resolution technique using a lidarsystem according to an embodiment is not limited to detection of hardsurfaces but is also capable of detecting and characterizing “softtargets” whose optical density becomes sufficiently large to producemultiple scattering effects in the media. For example, water quality isoften described by the level of turbidity. Turbidity is caused by thedistribution of suspended particles in the water that reach sufficientoptical density to scatter photons multiple times along the light path,consequently attenuating the light and obscuring the light path. These“soft targets” constitute a scattering target for a lidar system.

Lidar systems may work under a single-scattering assumption, that is,transmitted light scatters only once back to a co-aligned receiver. Inmany situations, this is an appropriate assumption to make but inoptically dense media, such as turbid water or clouds, it is impossibleto ignore the contribution of multiple scattering. Three of theimportant considerations that multiple scattering introduces is anincrease in the angular spread of returned photons (light will scatteraway from its initial direction axis and back to the detector),stretching the return in time (light travels an extended path within themedia before returning to the receiver), and increase the amount ofdepolarization in the returned light (light transmitted with onepolarization will return with a different polarization).

In particular, the single-scattering elastic backscatter lidar equationcan be written in a form:

${P(z)} = {\frac{K(z)}{z^{2}}{\beta(z)}e^{{- 2}{\gamma(z)}}}$

where P (z) is the received power from range z, K(z) is the instrumentand overlap function, β(z) is the backscattering coefficient, γ(z)=∫₀^(z)a(z)dz is the optical depth at range z, and α(z) is the extinctioncoefficient. Lidar systems exploit this equation and attempt to extractβ(z) and a(z) which often involves some assumption about the relationbetween the two. In order for this equation to remain valid, photonswhich are not exactly backscattered are assumed to be lost forever. Oncethe field-of-view (FOV) is large enough that multiply scattered photonsmay remain in the field-of-view and their contribution can be captured,then this equation must be modified in some way.

In simulations (e.g., Monte Carlo simulations), we have investigated a)how the angular spread of returned photons depends on recorded range, b)what new information this can provide about the particles doing themultiple scattering, and c) whether this can be incorporated into alidar transmitter/receiver using current technologies.

There are two main results that have come out of these simulations forapplication to wide field-of-view lidar systems. The first is that wecan calculate the optical depth of a cloud or of turbid media bymeasuring the depolarization of the returned lidar pulse, as long as thefield-of-view of the receiver is large enough to observe multiplescattering. As the laser pulse penetrates the cloud it is depolarized bymultiple scattering. The number of scattering events is related to thetotal optical depth (or the optical depth at a given range) so we canthen relate the amount of depolarization to the optical depth of themedium. The second result is that the observed angular spread of thereturned multiply-scattered light is related to the size of theparticles. In Mie theory, as the particle size increases, the angularwidth of the forward scattering peak decreases, and therefore we seethat the angular spread of the returned light is also smaller. By takingmeasurements with different field-of-view we can then work out how thelidar pulse is spreading laterally through the medium, which theninforms on the particle size. These two measurements, takensimultaneously, can serve to identify two important cloud parameters.

Most lidar systems operate in a single-scattering regime but in anoptically dense media such as a cloud or turbid water, this assumptionbreaks down. For sufficiently narrow laser divergence and receiver fieldof view, it is still possible to observe primarily single scatteredphotons. In a simulation, it is possible to separate returned photons byscattering order, returned angle, and exact range. This permits analysisof different types of scattering trajectories.

The easiest scattering order to analyze is the first order. Thesephotons are ones that undergo exactly 180° backscattering. Forscattering from spherical particles with linear incident polarization,the returned light will have the exact same polarization as thetransmitted light; spherical particles do not depolarize incident linearpolarization. Particles of different shapes and orientations willhowever depolarize perfectly backscattered light so depolarization isoften used, for example, to differentiate water clouds from ice-crystalclouds. As the optical depth increases, more total light is reflectedbut this light is also concentrated near the detector, the high opticaldensity attenuates the signal as photons are scattered away from theirdirect path. The total range of the single-scattered photons also stopsdirectly at the cloud boundaries.

As the field-of-view of the receiver is increased, the receiver becomessensitive to multiple-scattered photons. As the field of view increases,more light is allowed into the detector making a mist-cloud appearbrighter than a single-scattering assumption would predict. The cloudalso appears to extend beyond its physical boundary due to photonsrattling around inside the cloud, taking more time to return to thedetector.

Once photons are multiply scattered some depolarization can occur, andthe best way to view this is by opening the field-of-view. For a fixedwide field-of-view and spherical particles, separating returns which areco- and cross-polarized to the transmitted photons illustrate the changein depolarization due to multiple scattering. The co-polarized returnswill include both single- and multiple-scattered photons while thecross-polarized signal will only come from multiple scattering. Whenmore scattering occurs more of the returned light is depolarized. Thesingle-scattering events are dominant throughout the range but thehigher order terms become increasingly important at larger ranges. Thisis very dependent on the total field of view and total optical depth.

Multiple-field-of-view (MFOV) lidars may be used to analyze multiplescattering effects. These systems have methods to quickly vary the fieldof view of the receiving telescope to capture signals as a function ofrange on a short enough timescale to not be confused by changes in cloudstructure. One way that this works in practice is to use a rotating diskthat has different sized-holes drilled at a constant radius as the fieldstop in a telescope. As the disk rotates, different sized holes lead todifferent fields of view and by timing the rotation of the disk to thefiring of the laser it is possible to record very quickly signals fordifferent FOVs.

The need for multiple field-of-views is to analyze how the lidar pulsespreads transverse to its initial direction. In the total signalobserved, the small particles have a strong single-scattered return atthe smallest FOV, due to the wide shape of the phase function, thecross-polarized signal comes back from a very high FOV angle. For thelarge particles, due to the low backscatter in the phase function, thesingle scattered signal is a much lower percentage of the total signal.Also due to the very tight forward peak of the phase function much morecross-polarized signal comes from the lower FOV than in the smallparticle case.

The way in which the laser pulse spreads through the cloud depends onthe extinction coefficient and the particle size so the goal of a MFOVinversion scheme is to extract these two parameters. From theseparameters other physical quantities of interest such as liquid watercontent (LWC) and broadband extinction can be derived. When designing aMFOV lidar system, the choice of field-of-views is essential forgathering information.

The earliest method used to evaluate multiple scattering was to take aratio of the returns from the cross- and co-polarized channels calledthe linear depolarization ratio:

$\delta = \frac{I_{\bot}}{I_{❘❘}}$

Very generally, the more multiple scattering, the higher this ratio willbe. As discussed in earlier sections, this ratio will be dependent onthe experimental parameters such as FOV and particle size/density etc.Another metric which encapsulates similar information is the linearpolarization contrast:

$\delta_{LPC} = \frac{I_{❘❘} - I_{\bot}}{I_{❘❘} + I_{\bot}}$

The linear polarization contrast is constrained to lie between −1 and 1whereas the depolarization can take on any value greater than 0 (but isgenerally less than 1). As light travels through the cloud and scattersmore than once, the depolarization will increase and the linearpolarization contrast will decrease. For unpolarized light, or equalquantities of co- and cross-polarized signal, δ_(LPC)=0. The lightreturned in the smallest field of view maintains the initial linearpolarization since this light is scattered only once before returning tothe receiver. As the FOV increases δ_(LPC) decreases, reaching a valueof 0 beyond the physical boundary of the cloud.

Further, the azimuthal variation in the phase function suggests thatthere will be an azimuthal variation in the returned polarizationpatterns; the light in the two different polarization channels comesfrom different places in the cloud. From simulations, the initialpolarization is horizontal, and the majority of the scattering isperpendicular to the initial polarization plane, as expected from theforms of the phase functions. The co-polarized signals show a two-foldsymmetry around the polarization plane. The four-fold symmetry ispresent in the cross-polarized signals. These patterns have beenanalyzed semi-analytically and can be explained by the conservation ofangular momentum of light. This semi-analytical work has shown thatthere is information in these patterns regarding important microphysicalproperties of the cloud.

By comparing the co-polarized patterns returned from the small-particlecloud and from the large-particle cloud it is clear that there is someinformation on the particle size as the co-polarized signals are quitedifferent. Part of the problem with these azimuthal measurements is thedifficulty in experimentally recording these patterns as a function ofrange into a cloud. Most measurements made so far involve very widerange-bins that typically include an entire cloud depth and soinfluences due to variations in particle size and concentration areintegrated over and difficult to separate. Gated intensified chargecoupled devices (ICCDs) have allowed for this type of analysis.

In order to understand how the azimuthal patterns will evolve as afunction of range through the cloud it helps to first investigate theazimuthal patterns due solely to second-order scattering. Fromsimulations, the scattering from small particles shows a largedifference between the full patterns and the second-order patterns whichis because of contributions from higher scattering orders. The largeparticle full patterns on the other hand are quite similar to the secondorder patterns. The phase function for the large particles is heavilypeaked in the forward direction meaning that photons leave the cloudafter fewer interactions than they do in the small-particle case. Thismeans that optical depth does not always track well with the number ofscattering events.

For the azimuthal dependence of the co- and cross-polarized signals,Semi-analytic methods to interpret doubly-scattered photons suggest thatthe azimuthal dependence of the cross-polarized returns should follow a1−cos(4φ) dependence. This suggests that these signals can be fit to anequation of the form:

I(φ)=A+B cos(4φ)

where A>0 and B<0. The ratio between minima and maxima can then bedefined as:

$R = {\frac{I_{\min}}{I_{\max}} - \frac{A + B}{A - B}}$

The ratio for double-scattered photons would be zero. Higher orderscattering will blur the strong azimuthal patterns. This contrast may beused to correlate the azimuthal contrast of the cross-polarized channelto the optical depth. The change in contrast comes about throughmultiple scattering so perhaps a better way to analyze it is tocorrelate the contrast with a number of scattering events.

Rather than take the contribution of all photons or onlydouble-scattered photons, it is possible to sort the returned photonsinto range-bins and thereby fit the azimuthal dependence as a functionof range. For all particles, as the range into the cloud increases theratio increases from near-zero at the cloud boundary to higher valuesdeeper into the cloud. The difference between the particles comes aboutbecause the number of scattering events in clouds with the same opticaldensity is lower for large particles. This is because of thehighly-forward-peaked nature of the phase function for scattering fromlarge particles. The azimuthal contrast ratio compares very well to theaverage multiple-scattering number, up to around three scatteringevents. By using the azimuthal contrast ratio we can determine whethermost particles in a given range-bin have been single-, double-, ortriple-scattered.

This connection between number of scattering events and azimuthalcontrast can be used also to measure the optical depth of the medium.Measuring the azimuthal contrast of the cross-polarized signal is a goodmeasure of the optical depth of the cloud. Note that this will changefor different particle sizes because of the propensity for largeparticles to primarily scatter in the forward direction.

Given the difference in co-polarized images for scattering fromparticles with different size parameters, the co-polarized azimuthalcontrast can be fitted to a form:

I(φ)=A+B cos(2φ)+C cos(4φ)

A contrast which measures the intensity difference in two- and four-foldpatterns can be defined as

$R = \frac{C + B}{C - B}$

This ratio shows a clear relation to particle size. This method hasfurther promise for providing an additional measure of particle sizeindependent from measures of optical density.

As such, the sub-pulse width resolution technique using a lidar systemaccording to an embodiment can be used to exploit these attributes ofmultiple scattering to characterize turbid media. In an embodiment, onemethod is an invasive approach where a target is lowered into the liquidmedia serving as a scattering target for the lidar. Frequent recordingsof high-precision range and intensity of the backscattered signal as thetarget is lowered through the liquid is then afforded by the sub-pulsewidth resolution technique using the lidar system. A highly preciserange profile of backscattered intensity through the liquid column canbe determined, enabling estimates of the extinction coefficient of theturbid media and an overall determination of optical depth.

In an alternate embodiment as opposed to the invasive measurement asdescribed above, another method is a non-invasive measurement of thedepolarization ratio of the turbid media. Multiple scattering bysuspended particles will modify the original polarization, quantified bythe amount of depolarization, that allows for differential detection ofsingle- and multiple-scattered photons from optically dense media. Theoptical depth of a clouded or of turbid liquid media can then bedetermined by measuring the amount of depolarization of the returnedlidar pulse, as long as the field-of-view of the receiver is largeenough to observe multiple scattering. As the laser pulse penetrates theoptically dense media, it is depolarized by multiple scattering. Thenumber of scattering events is related to the total optical depth (orthe optical depth at a given range), which can then be relate to theamount of depolarization to the optical depth of the medium.

Furthermore, the observed angular spread of the returnedmultiple-scattered light is related to the size of the particles anddiffers spatially for different planes of polarization. By takingmeasurements using the sub-pulse width resolution technique using thelidar system with different fields-of-view, it can be worked out how thelidar pulse is spreading laterally through the medium, which theninforms on the mean particle size distribution of the suspendedparticulates. Combining the depolarization measurement with the multiplefield-of-view measurement can serve to identify two important parametersof turbid media: optical depth and particle size. A way to capturesimultaneous returns within multiple fields of view is to image themultiple scattered signal on a planar imaging array. In an embodiment,5-6 different fields-of-view may be needed. In another embodiment, thesuitable number of fields-of-view would be highly dependent on chosenparameters such as cloud distance, cloud depth, particle density,particle size, etc.

Compact and scalable sensor arrays with each pixel registering its ownsignal are available in, e.g., silicon photomultipliers. As each sensorpixel represents a physical location of the scattered signal from atarget, the lateral spreading caused by multiple scattering in turbidwaters can be determined. In an embodiment, this system configurationmay be realized in a lidar system. For example, PMT detectors 435 and436 in the lidar system 400 (as discussed with respect to FIG. 4 ) maybe replaced with silicon photomultiplier arrays or other sensor arrays.

Particularly, in an embodiment, a charge couple device (CCD) camera maybe used for capturing the scattered signal. However, traditional CCDcameras may not have the time-sensitivity that would allow forrange-resolved measurements. The use of gated intensified CCD (ICCD)cameras may overcome this issue as ICCD cameras have the capability tobe gated in time, allowing for the measurement of very narrow (ns) timeslices. For each laser shot, only a single time slice could be measuredand many laser pulses would have to be integrated to produce a finalimage for that particular time slice. Measurement in this way of a fulldepth profile could be time consuming depending on the signal levelsreturned from the cloud.

In another embodiment, the use of a spatial mask placed at a focal planeof the receiver may allow for measurement of azimuthal signal. Lightthat passes through the mask would all be focused onto a single-elementdetector such as a photomultiplier tube (PMT) or a photodiode. Thisspatial mask could be designed to isolate different parts of theexpected pattern, allowing for measurements of the contrast. By rotatingin different spatial masks into co- and cross-polarized channels andrelying on symmetries present in the images, determination of azimuthalcontrast would be possible. This method may allow for at least animperfect measure of the contrast which will in turn provide informationon the optical depth of the medium.

In yet another embodiment, a similar mask can be created for measuringthe co-polarized variations, although designing the mask is somewhattrickier due to the subtle differences in scattering profiles betweenparticles of different sizes. The larger particles lead to a muchstronger peak along the horizontal axis (φ=0°, 180°) compared to smallerparticles. With more realistic (larger) droplet sizes, these differencescould change significantly.

Thus, a non-invasive, lidar remote-sensing technique for determiningoptical depth and particle size in turbid media can be developed byexploiting the effects of multiple scattering and using the sub-pulsewidth resolution technique using the lidar system according to anembodiment to determine detailed estimates of depolarization and angularspreading. In another embodiment, by combining the invasive andnoninvasive approaches, using the lidar technology, a calibrationprocedure can be developed that relates the remote observation ofoptical depth and particle size with the invasive measurement ofextinction and optical depth.

Referring to FIG. 8 , the lidar system 800 in an alternate embodimentincorporating the features as discussed above for detecting andcharacterizing targets with sufficiently large optical density thatproduces multiple scattering effects in the media. The lidar system 800may be modified from the lidar system 400 by replacing the PMTs 435 and436 with sensor arrays 835 and 836. In an embodiment, the field stop 832may be further opened (or completely opened) compared to the field stop432 for a larger field-of-view for observing the multiple scatteringlight. In a further embodiment, the processor 843 may include processes,algorithms, and/or other calculations as calibrated to the lidar system800 for determining data and information related to the multiplescattering effects in the media, for the invasive approach, thenoninvasive approach, and/or a combination of the invasive andnoninvasive approaches. In one embodiment, multiple fields of view canbe obtained simultaneously or substantially simultaneously.

In further detail, the lidar system 800 includes light transmitter 810,light receiver 830, and timing electronics 840. In this embodiment, thelight transmitter 810 includes laser 812, beam expander 813, half-waveplate 814, polarizer 815, and prisms 816. Laser 812 acts as a lightsource for lidar system 800 and is configured to emit a focused light asthe basis of the transmitted light signal. Laser 812 can be a pulsedlaser, continuous wave (CW) laser, polarized laser, or other types oflasers. In other embodiments, laser 812 can generically include otherlight sources as known in the art (i.e., lamp or LED light). In oneembodiment, a 850 ps pulsed laser is used as laser 812. Beam expander813, half-wave plate 814, polarizer 815, and prisms 816 are optional andare configured to focus and align the transmitted light signal towardstarget 820. In this embodiment, the chain of beam expander 813,half-wave plate 814, polarizer 815, and prisms 816 are each aggregatedand aligned to the optical path of the transmitted light signal. Beamexpander 813 is configured to expand the transmitted light signal fortight spot targets. Half-wave plate 814 may be mechanically orelectrically (i.e., using a liquid crystal variable retarder) operableto control the retardance of the focused light signal along the opticalpath. Polarizer 815 is configured to polarize the light signal with aknown polarization. A polarizing laser may also be used as laser 812 fora known polarization. Prisms 816 are configured to coaxially direct thetransmitted light signal to target 820 as known in the art.

In operation, trigger 811 may be electrically coupled to laser 812 orother components of light transmitter 810 to start the transmission ofthe light signal. In other embodiments, light transmitter 810 mayoperate continuously without trigger 811. Light signal is transmittedfrom light transmitter 810 to target 820. Target 820 includes at least afirst surface and a second surface as described herein. The transmittedlight signal is partially scattered from the first surface as a firstscattered light signal and partially refracted into the target. Therefracted light is scattered from the second surface as a secondscattered light.

The first scattered light signal has substantially the same polarizationas the transmitted light signal while the second scattered light signalwill have a different polarization due to the scattering from the secondsurface. The first and second scattered light signals may have anoverlapped interpulse portion forming one combined scattered lightsignal.

Light receiver 830 includes telescope 831, field stop 832, spectralfilter 833, splitting polarizer 834, first detector 835, and seconddetector 836. Each of these components are aggregated and aligned to anoptical path of the scattered light signal. Telescope 831 acts tocollect the scattered light signal. Field stop 832 and spectral filter833 are optional components. Field stop 832 acts to limit the field ofview of light receiver 830 where the scattered light signal would begathered. In this embodiment, field stop 832 may have a widerfield-of-view than field stop 432 or may be completely open. Spectralfilter 833 acts to further filter the received light to the lightspectrum of interest (e.g., limiting the spectrum to the expectedfrequency of the scattered light signals).

Splitting polarizer 834 acts to separate the received scattered lightsignal according to the polarization. In this embodiment, thepolarization splitter 834 is aligned with the optical path of thescattered light signal. As scattered light signal reaches polarizationsplitter 834, the cross-planar polarized component of the signalsubstantially passes through polarization splitter 834 while theco-planar polarized component of the signal substantially reflects. Theangle of reflection is a function of the type of polarizer used (i.e.90° angle or 62° angle for a Glan Taylor polarizer). Here, the firstscattered light signal scattered from the water surface containingcross-planar polarized light is substantially reflected (i.e., at a 90°angle) while at least the co-planar polarization component of the secondscattered light signal scattered from the water floor containingdepolarized light is substantially transmitted. Other orientations arealso possible depending on the polarization methodology used on thetransmitted light signal and the type of polarizer used for polarizationsplitter 834.

Detector 835 is positioned at a 180° optical path from the reflectedlight signal and configured to detect the cross-planar polarizationcomponent of scattered light signal. Detector 836 is positioned at theoptical path of the reflected light signal (i.e., 90°) and is configuredto detect the co-planar component of the scattered light signal.Detectors 835 and 836 may each include a sensors array e.g., siliconphotomultipliers, for detecting scattered signal from the target (e.g.,multiple scattered light signal).

It is noted that polarizing beam splitter 834 can be positioned at avariety of angles to split the scattered light signal at other angles.Detectors 835 and 836 can be positioned at other configurations toreceive such split components of the multiple scattered light signals.

The timing electronics 840 may include a constant fraction discriminator(CFD) 841, time-to-digital converter (TDC) 842, and processor 843.Processor 843 is coupled to CFD 841 and TDC 842 through a control linefor control and feedback of these components. CFD 841 is coupled todetectors 835 and 836 through a signal conditioning line and isconfigured to output an apex of the photon count signal at certainintervals representing the time at which the signal has meaningfullyarrived. In one embodiment of the invention, CFD 841 has an 8 nsresolution. TDC 842 is coupled to CFD 841 and is configured to convertthe time signal output by CFD 841 into a digital signal. In oneembodiment of the invention, TDC 842 has a resolution of 27 ps.

Processor 843 is coupled to TDC 842 and is configured to take thedigitized timing signal and determine the time of arrival of eachcomponent (co-planar and cross-planar polarized signals in oneembodiment) and calculate the difference in the time of arrival of thetwo signals. In this embodiment, the processor 843 may further includeprocesses, algorithms, and/or other calculations as calibrated to thelidar system 800 for determining data and information related to themultiple scattering effects in the media, for the invasive approach, thenoninvasive approach, and/or a combination of the invasive andnoninvasive approaches.

Further, an initial calibration to lidar system 800 may be neededbecause the light paths to detectors 835 and 836 may not be the sameafter the scattered light is separated by polarizing splitting 834.According to one embodiment, this calibration can be accomplished byusing a scattered signal from a surface that is depolarizing and notinga difference in the assessment of distance to that surface between thedetectors 835 and 836. The difference in the assessment of distance islikely due to the slightly different optical paths between each ofdetectors 835 and 836 and polarizing splitter 834. In one embodiment,this calibration can be performed once and saved for adjustment byprocessor 843. The correction and adjustment can be applied tosubsequent depth data by processor 843.

One embodiment is directed towards using a lidar system and techniquesdescribed herein to classify skin of a patient. The pathway of lightinto the skin is believed to have several pathways. Skin includesseveral layers as known in the art including epidermis, dermis andsubcutis/hypodermis. It is believed that the lidar system herein andtechniques, for example, as described with Example 9 can be used toidentify normal skin pathology and abnormal skin pathology. The abnormalskin pathology may include cancerous or precancerous cells. The lidarsystem can be utilized with an imaging technique as described withreference to Imaging Skin Pathology with Polarized Light, Journal ofBiomedical Optics, 7(3), 329-340, July 2002, which is herebyincorporated by reference as if fully set forth herein. Other lidarmeasurement techniques are described with regard to examples 1-9 hereinand it is believed that these techniques can be utilized for determiningvarious skin pathologies.

In one embodiment, the lidar system can be configured to scan thesurface, collect parallel and perpendicular polarizations and recordintensities. It is believed the pathology of the cell can bedifferentiated by different wavelengths, volume of cell structures,density of cell structures and other characteristics of the systemand/or cells. It is also believe the system can penetrate to at least 4mm.

FIG. 10 illustrates an exemplary block diagram of a communicationnetwork for use with embodiments of the invention

In one embodiment, the system 1000 is utilized for communication andprocessing techniques and methods described herein. The system can beconfigured with large-scale, parallel computational capabilities thatare leveraged for image and signal processing demands of embodimentsherein. In embodiments herein, lidar systems are utilized for variousmethods and systems for remote sensing measurements, and particularly tomethods and systems for remote sensing measurements throughsemi-transparent media, e.g., glass, plastic, sapphire, and combinationsthereof, and even more particularly to methods and systems for sensing arelative distance and/or shape of an object or feature in variousenvironments and is configured to work with the system 1000. Optionallyand/or alternatively, the lidar system is part of a larger navigationsystem can be configured to work with the system 1000. In thisembodiment, the lidar systems may be in communication with any aspect ofthe system 1000.

Referring to FIG. 10 , the system 1000 includes one or more networks,including wide-area network 1002, e.g., the Internet, company ororganization Intranet, and/or sections of the Internet (e.g., virtualprivate networks, Clouds, and the Dark Web), and local-area network1004, e.g., interconnected computers localized at a geographical and/ororganization location and ad-hoc networks connected using various wiredmeans, e.g., Ethernet, coaxial, fiber optic, and other wiredconnections, and wireless means, e.g., Wi-Fi, Bluetooth, and otherwireless connections. The system 1000 can include a number of networkdevices 1006, 1008, 1010, 1012, and 1014 that are in communication withthe other devices through the various networks and/or through othermeans, e.g., direct connection through an input/output port of a networkdevice 1016, direct connection through a wired or wireless means, andindirect connection through an input-output box, e.g., a switch.

Network devices 1006, 1008, 1010, 1012, and 1014, which may also connectthrough the networks 1002 and 1004 using various routers, access points,and other means. For example, network device 1010 wirelessly connects toa base station 1018, which acts as an access point to the wide areanetwork 1002. The network devices may include a I/O, processor, memoryand storage configured to perform processes and methods of modulesdescribed herein and known in the art. The processing for the lidarsystem or navigation system may be done in parallel, on a network devicein the cloud, and combinations of the same. Base station 1018 may be acellular phone tower, a Wi-Fi router or access point, or other devicesthat allow a network device, e.g., wireless network device 1010, toconnect to a network, e.g., wide area network 1002, through the basestation 1018. Base station 1018 may be connected directly to network1002 through a wired or wireless connection or may be routed throughadditional intermediate service providers or exchanges. Wireless device1010 connecting through base station 1018 may also act as a mobileaccess point in an ad-hoc or other wireless network, providing accessfor network device 1014 through network device 1010 and base station1018 to network 1002.

In some scenarios, there may be multiple base stations, each connectedto the network 1002, within the range of network device 1010. Inaddition, a network device, e.g., network device 1010, may be travellingand moving in and out of the range of each of the multiple basestations. In such case, the base stations may perform handoff procedureswith the network device and other base stations to ensure minimalinterruption to the network device's connection to network 1002 when thenetwork device is moved out of the range of the handling base station.In performing the handoff procedure, the network device and/or themultiple base stations may continuously measure the signal strength ofthe network device with respect to each base station and handing off thenetwork device to another base station with a high signal strength tothe network device when the signal strength of the handling base stationis below a certain threshold.

In another example, a network device, e.g., network device 1014, maywirelessly connect with an orbital satellite 1019, e.g., when thenetwork device is outside of the range of terrestrial base stations. Theorbital satellite 1018 may be wirelessly connected to a terrestrial basestation that provides access to network 1002 as known in the art.

In other cases, orbital satellite 1018 or other satellites may provideother functions such as global positioning and providing the networkdevice with location information or estimations of location informationof the network device directly without needing to pass information tothe network 1002. The location information or estimation of locationinformation is known in the art. The network device may also usegeolocation methods, e.g., measuring and analyzing signal strength,using the multiple base stations to determine location without needingto pass information to the network 1002. In an embodiment, the globalpositioning functionality of the orbital satellite 1018 may use aseparate interface than the communication functionality of the orbitalsatellite 1018 (e.g., the global position functionality uses a separateinterface, hardware, software, or other components of the network device1010 than the communication functionality). In another embodiment, theorbital satellite with the global position functionality is a physicallyseparate satellite from the orbital satellite with communicationfunctionality.

In one scenario, network device, e.g., network device 1008, may connectto wide area network 1002 through the local area network 1004 andanother network device, e.g., network device 1020. Here, the networkdevice 1020 may be a server, router, gateway, or other devices thatprovide access to wide area network 1002 for devices connected withlocal area network 1004.

In one scenario, users may have access to navigation or lidar systemdata over the system 1000. In this embodiment, the users could pay foraccess to the data on the system 1000.

EXAMPLES

Without intending to limit the scope of the invention, the followingexamples illustrate how various embodiments of the invention may be madeand/or used.

Example 1

A simulation of the normalized received intensity for targets of varyingdegrees of depolarization d is illustrated in FIG. 6A. The sinusoidalnature of received light from a polarization preserving target 513(curve labeled d=0) is evident, while the detector 536 registers aconstant intensity of 0.5 for a completely depolarizing target 513(curve labeled d=1). By translating the quarter-wave plate 528 from anorientation θ of π/4 radians to θ of 0 radians, scattered signals aremodulated between polarized water surface and volume of the water bodyreturns and depolarized floor returns.

Bathymetric measurements were made at the University of Colorado,Boulder, using a lidar system 504 as illustrated in FIG. 5 . Thetransmitter consisted of a CW-diode pumped, passively Q-switched Nd:YAGmicrochip laser. The laser outputs 2.45 microjoule of linearly polarized532 nm light at a repetition rate of 14 kilohertz and pulse width of 450picoseconds. A half-wave plate aligned the laser 508 light polarizationto the vertical transmission plane of a 532 nm PBS. Light exiting thePBS was transmitted through a rotatable quarter-wave plate toward acontrolled target consisting of a column of water on top of apolarization-altering floor substrate. Scattered laser light received bythe instrument was collected with a detector comprising aphotomultiplier tube in photon counting mode. The output PMT voltage wasanalyzed on an oscilloscope with 550 ps timing resolution and stored forpost-processing.

Data acquired during reception of scattered signals from the target for3 centimeter deep water as measured physically, are presented in FIG.6B. The quarter-wave plate was positioned in θ orientations of π/4 and 0radians. The received intensity from the PMT analog signal illustratesreduction of polarized signals from the water (dotted-solid) when thequarter-wave plate fast axis is aligned to the vertical PBS polarizationtransmission plane to measure polarization-altered floor signals(solid).

The experiment was repeated using a digital lidar receiver with 27picosecond timing resolution, as illustrated in FIG. 6C. To illustratethe ultimate resolution of the timing unit, data were again taken for 3centimeter (solid) and 1 centimeter (dashed) water depths.

Taking into account the refractive index change of water n relative toair, water depth h is calculated as:

$\begin{matrix}{h = \frac{c\Delta t}{2n}} & (8)\end{matrix}$

where the time delay Δt is evaluated by differencing the FWHM points(horizontal dashed) of the surface and floor curve trailing edges. Theresults presented in FIG. 6C produced depth measurements d of 2.7 cm and1.2 cm. The 27 ps resolution of the timing unit imposes a 3 mmuncertainty on the water depth estimate. Therefore, the observed depthsare well within the uncertainty of the measurement.

Transmission of vertically polarized light through the PBS andtranslation of the quarter-wave plate orientation modulates receivedsignals between polarization preserving water surface and body returnsand polarization-altered floor scatter. By removing water surface andcolumn effects through polarization modulation, bathymetric ambiguitiesbetween water surface and body floor returns are negated. As a result,the fundamental lower limit on shallow water bathymetry imposed bysystem bandwidth limitations is reduced beyond traditional techniquesusing a single detection channel. The technique presented here hasdemonstrated resolution of 1 cm water depth.

Although embodiments described above discuss the inclusion of a lightsource comprising a laser, it should be appreciated that the lightsource is not required to comprise a laser. Moreover, transmission andreception of light may be through separate apertures. According to suchembodiments, a polarizing beam splitter need not be included. Forexample, the reception channel can instead include a filter or otherpolarization discrimination element. Moreover, although examples havediscussed the transmission of light having particular polarizations,other polarizations can be used. In particular, it is sufficient totransmit polarized light, and receive polarization-altered light fordiscriminating polarization preserving and polarization-alteringscattered light. In addition, although methods and systems herein havediscussed the disclosed polarization techniques in connection withlidar, embodiments of the present invention also have application toguided wave optics, optical time domain reflectometry, fiber opticsensor networks, and/or other applications as known now or may be laterderived.

Example 2

FIG. 7 illustrates the result of an experimental setup measuring depthof semi-transparent media with sub-pulse width resolution. Here, a laserpulse width of 450 ps was used, corresponding to a pulse length (range)of 6.75 cm. A piece of glass with polarization preserving andsemi-transparent surfaces and with a thickness of 1.4 cm was placed infront of a polarization-altering (depolarizing) wall at a distance of3.4 cm. A lidar system, similar to lidar system 400 according to oneembodiment of the invention, was placed at a distance of 30 m from themedia.

First, a control measurement was made with the piece of glass removed.The dotted lines in the graph show the relative distance of the wallfrom this control measurement. Both the co-planar and cross-planarpolarization components in this measurement are scattered from the walland register the same distance after being calibrated (dotted lines).

Next, a measurement is made with the glass setup as described. The solidlines in the graph show the relative distance of the glass and the wallfrom this measurement. The co-planar polarization component is scatteredfrom the glass surface. The cross-planar polarization component isproduced by scattering from the wall. The distance from the glasssurface to the wall can then be determined based on previous embodimentsof the invention. Further, in this measurement, the cross-planarpolarization component scattered from the wall is further delayed by therefraction index of the glass. Therefore, a measurement on the thicknessof the glass can also be determined by this delay in this secondmeasurement.

Both the distance from the glass surface to the wall and the thicknessof the glass can be determined using both the measurements from thecontrol experiment and with the glass setup. For the distance of theglass first surface to the wall, since the cross-planar polarizationcomponent is scattered off the glass surface in the experiment with theglass setup, the distance is the difference between the wallmeasurements in the control experiment and the cross-planar polarizationcomponent in the experiment with the glass setup. The calculateddistance is 3.7 cm±0.4 cm (actual measured distance from wall to firstglass surface is 3.4 cm±0.1 cm). For the thickness of the glass, sincethe co-planar polarization component is scattered off the wall in theexperiment with the glass setup and further includes the delay by therefraction index of the glass, the thickness is the difference betweenthe wall measurements in the control experiment and the co-planarpolarization component in the experiment with the glass setup. Thecalculated thickness is 1.4 cm±0.4 cm. Comparing the result of thecalculated distance (3.7 cm) and thickness (1.4 cm) with the pulse widthof the laser (6.75 cm), both the distance and the thickness measurementsare confirmed to be at sub-pulse width.

Example 3

FIG. 8 illustrates the result of an experimental setup measuring waterclarity with sub-pulse width resolution. Here, the lidar systemoperating at 532 nm was positioned 7 ft above a tank of water andpointed down into the water. The tank of water also contained 1.5 cu.ft. of sand, which could be stirred up in order to introduce turbidityinto the water. The water was 25 cm deep above the sand bottom surface.This experimental setup demonstrated that the invasive lidar techniqueis able to measure extinction through turbid media.

First, a second green laser system was used to determine the verticaldistribution of turbidity in the water to check on the uniformity of theturbidity through the water column. This laser system was directedhorizontally through the water tank to a detector to record theintensity of light that passed through the water. As the density ofsuspended sand was increased through stirring, the detector sensed alarge decrease in laser light that was transmitted through the water. Byvertically translating this horizontally oriented laser/detector system,it was confirmed that the amount of transmitted light was independent ofthe vertical position of this laser on the time scale of a few minutes.This suggests that the density of suspended sand was independent ofvertical position and uniformly distributed through the water column.

Second, the sub-pulse width resolution technique using a lidar systemaccording to an embodiment was applied to the fully mixed turbid waterby positioning the lidar above the tank. A scattering target (a block ofwood) was attached to a rod and progressively lowered into the tank ofturbid water. At 2 cm intervals the target was held steady in the water,and the peak intensity of the backscattered laser light off the targetwas recorded on an oscilloscope, bypassing the acquisition systemdescribed in FIG. 4 (e.g., CFD 441 and TDC 442) and FIG. 8 (e.g., CFD841 and TDC 842) in order to achieve a more direct measurement of theintensity.

As the scattering target was lowered deeper into the water, the returnedsignal decreased due to the turbidity level of the water. Using a narrowfield-of-view and observing the change in intensity with range, atwo-way extinction measurement of k=0.25 cm⁻¹ was calculated by fittingthe measurements to an exponential function e^(−kx), as seen in FIG. 9 .This two-way extinction coefficient corresponds to a one-way extinctionthrough the water of k=0.25/2=0.125 cm⁻¹. This is much larger than theabsorption coefficient of water for the 532 nm laser light (˜0.001cm⁻¹), indicating that the extinction observed is largely due to thescattering of light from the suspended fine-grain sand present in thewater. This measurement provides a direct means of determining the levelof turbidity of the water and can be used as important information indetermining the multiple scattering attributes of optically dense media.

In this example, calm but turbid water conditions were emulated in thelab. The main concern is the achievable depth for a given amount ofextinction along the laser beam's water path. Turbid water includessmall particles suspended in the water. Working in shallow waters ofless than a few meters does have its benefits as the level of extinctionby the water is significantly reduced leading to high signal-to-noise(SNR) signals for determining surface and bottom positions. Of course,particulates suspended in the water can add to the extinction byscattering and absorption as their number density increases, leading tolevels of turbidity that may be enough to prohibit signals from reachingthe bottom.

Because the lidar system and techniques have such high depth resolutionless than about one centimeter, this example highlights turbiditymeasurement capabilities of the lidar system and techniques, or areflecting transmissometer, that can determine the total extinctionalong the path and relate that extinction to the maximum detectabledepth. By lowering a scattering target through the water at intervals ofabout 2 centimeters, a profile of extinction was retrieved—essentiallyapplying our novel technique to create a reflecting transmissometer.This is illustrated in FIG. 9 for highly turbid water produced in thelab. A two-way extinction value of 0.25 cm⁻¹ was derived by fitting theobservations to an exponential. This value is ten times more turbid thanmost lakes and rivers. Although this experiment shows a limiting depthof about 15 cm in this highly turbid case, this is not the limitingdepth of the system because the system was not optimized for maximumdepth during the experiment. These observations enable quantifiableestimates of turbidity to test light propagation and opens thepossibility of using the Lidar technique to provide water opacitymeasurements.

Example 4

FIG. 11A illustrates an experimental setup of Example 4 configured tomeasure the height of at least two surfaces in calm and substantiallyclear water. In this example a lidar system configured to map the heightof at least two surfaces. The experimental setup used was an aquarium1100 shown in FIG. 11A. The aquarium 1100 was filled with about 15gallons of water 1104 and partially filled with 100 pounds of QuickretePlay Sand 1102. The sand 1102 was arranged to have a slope of about 35degrees with a portion of the sand in and out of the water 1104 theslope was configured to mimic an entrance to a body of water, e.g.,ocean or lake. A first rock 1108 and second rock 1110 are arranged on alower portion of the sand 1102. The first rock 1108 has a diameter ofabout 5 cm and the second rock 1110 has a diameter of about 3 cm. Theserocks are spaced about 5 cm apart from each other.

In this example, the lidar system configured to map the height of atleast two surfaces includes a lidar system is described with referenceto FIGS. 4 and 8 . The system was arranged about 7 feet above the top ofthe tank.

The system includes a pulsed laser (TEEM Laser SNG-03E) operating at 532nm at 8 KHz repetition rate with 0.5 ns pulse width, with highlyvertical polarization (ThorLabs glan-thompson polarizer GT15A), atelescope receiver (made up of a ThorLabs AC508 achromatic lens and aThorLabs AC254 achromatic lens) with a polarization beam splitter(ThorLabs PBSW-532 and Thorlabs GT15A) and two PIN photodiode detectors(ThorLabs Det10a), one detecting co-polarized light while the otherdetects cross-polarized light. The acquisition system is described inFIG. 4 (e.g., CFD 441 and TDC 442) and FIG. 8 (e.g., CFD 841 and TDC842) in order to achieve a more direct measurement of the intensity. Thelidar system can provide about ±3 mm uncertainty in depth while nadirpointing, even though the effective range resolution of the laser pulsewas about 7 cm and detector bandwidths were about 40 cm. In operation,photon-induced electrical pulses from each polarization state are passedthrough the CFD to produce a digital pulse for the TDC. The CFD/TDCoperation provides precise and stable time tagging (a few picoseconds ofjitter) with first-in, first-out (FIFO) mode of acquisition such that atime tag for each detected pulse is recorded in real time. This resultsin sub-nanosecond time resolution, low data rates, and significantflexibility in post data handling. This can be applied either in photoncounting or Geiger modes. Acquisition dead time is not an issue as weare detecting surface and bottom return pulses on separatedetector/acquisition paths.

Referring to FIG. 11B, the maximum depth of water in the aquarium wasabout 34 cm. Using the lidar technique described herein, the top andbottom surfaces were resolved with an accuracy of better than acentimeter. The raw data from this experiment are shown in lower plot ofFIG. 11B. The presence of the beach is indicated in both theco-polarized and cross-polarized detector channels but as water becomespresent the signals deviate. The co-polarized channel registers thewater surface, shown by line 1114, and the cross polarized channelregisters the sandy bottom surface line 1116. The y-axis scale is incentimeters and we are able to discern features on the order of about 1cm or less. The laser pulse width is about 0.5 ns, or a length of about14 cm in water. We are able to achieve resolution less than a durationof the pulse from the lidar system using our technique. This far exceedscurrent lidar capabilities and, for all known existing systems, thistank of water would not even be detected as a water body. Fine detailscan be observed of the sandy bottom by line 1116 at objects 1118 and1120. The identification and relative size of the two rocks 1118 and1120, respectively, is clearly evident in FIG. 11B.

In operation the lidar system was placed 7 feet above an aquarium 1100,oriented with the laser pointed directly down at the aquarium (the laserlight 1112 can be seen interacting with the water). Horizontaltranslation of the system enabled longitudinal mapping of the scene. Thetransition from land to shallow water can be identified in the datapresented in the bottom portion of FIG. 11B, and the first rock 1108 andsecond rock 1110 placed in the aquarium are mapped with sub-centimeterresolution. Referring now to FIG. 11B, the results of the scan are shownin a plot having an on the y-axis height in cm and x-axis havingdistance in cm. The surface of the sand 1102 has profile 1116 it clearlyillustrates the profile of the first rock 1118 and the profile of thesecond rock 1120 are shown. Moreover, the profile of the surface of thewater is shown 1114. Finally, the maximum height 1122 of the water isshown as 34 cm.

As shown in FIGS. 11A and 11B, the shallow water lidar system providedmeasurements of two surfaces, the water surface and the sand surface,under clear and calm water conditions. The results are precise to lessthan 1 cm, and a comparison to measurements with a ruler placed in thewater reveal accuracy also less than 1 cm. The two rocks 1108 and 1110are measured with a ruler to have about a 5 cm height and about a 3 cmheight, respectively, and the lidar system also measures them at 5 cmand 3 cm above the sandy surface. We achieved sub-centimeter horizontalresolution with slow translation of the lidar horizontally across thescene. This example demonstrates capability of the lidar system whenmeasuring two or more surfaces at centimeter vertical resolution instill water. Any bottom object can be discerned with centimeterhorizontal resolution. The detailed measurement enables mapping ofsubmerged objects on bottom surfaces.

Example 5

This example illustrates a lidar system configured to map the height ofat least two surfaces in a slow flowing outdoor aqueous environment1200. The experimental setup used was operating a lidar system 1202described with reference to Example 4 over a slow flowing shallow streamin Boulder, CO as shown in FIG. 12A. The lidar system 1202 also includedan Inertial Measurement Unit (IMU) (VectorNav VN-200), which wasattached to the lidar system on a fixed mount bolted to an aluminumplate, to record changes in pointing and position. An image of submergedobjects is given in FIG. 12B.

On the bottom of the stream was at least a concrete block 1204 placed inthe stream and natural vegetation of plants 1206. In this example, theheights of at least two surfaces were mapped with a lidar systemincluding operating the lidar and the IMU together. In operation, thelidar system 1202 was rotated by hand to map out the stream bathymetryand identify submerged objects of plants 1206 and the concrete block1204. More specifically, as the lidar system 1202 was tilt-scanned wemapped out the underwater topography.

The top surface of the smooth water surface was found by pointing thelidar in a near-nadir direction and measuring time of flight for topsurface and bottom surface reflections simultaneously. Using the lidarfor off-nadir data and the position and pointing data from the IMU, wecreated a point cloud of the returned pulses. The IMU recorded “pitch”and “roll” of the lidar sensor, which can be interpreted as an altitudeand azimuth. The lidar records ranges to two surfaces. Range and twoangles define locations in the spherical coordinate system, which can beconverted to XYZ coordinate system using standard spherical-to-Cartesiantransformations. This was used to convert the measurements into a lidarpoint cloud referenced at the lidar position. This point cloud displaysthe concrete block and some nearby plants, and the resulting data hasbeen used to measure the size and shape of the concrete block asdescribed herein.

Referring to FIG. 12C illustrates a graph with a line 1208 showing thecross-section highlighting the submerged concrete block 1210 the graphincludes a depth (m) on the y-axis and cross-stream (m) on the x-axis.Referring to FIG. 12C, shows a three-dimensional map with a cross streamx-axis (m), downstream y-axis (m) and depth z-axis (m), that reveals theconcrete block 1212 in the foreground as well as plants 1214 that areshown in the upper part.

The precision of the measurement in FIG. 12C is 1 cm or less in height.Due to inaccessibility of the water we were unable to measure accurateruler measurements in this location. The horizontal resolution isdisplayed in FIG. 12D and was limited by the handheld nature of thetest. Along the lidar track, as shown in FIG. 12C, the resolution isgreater than 1 cm. The tracks were made to be roughly 5 cm apart.

This example demonstrates the applicability of the technique in outdoorsenvironmental conditions. The combination of the lidar and IMUmeasurements provide centimeter resolution in the vertical andhorizontal directions when the object is submerged in over 1 meter ofwater.

Example 6

This example illustrates vertical and horizontal mapping with a lidarsystem providing accurate range to the surface and bottom, water depth,and subsurface mapping of various features in a flowing river. Theexperimental setup included a lidar system 1302 described with referenceto Example 4.

The lidar system 1302 was mounted on a scanner 1304 over a flowing riverscene setup 1306 as shown in FIGS. 13B and 13F. The lidar system 1302was positioned three feet above the water surface pointing down at thewater on a scanner 1304 (custom scanner built by USGS personnel fromVelmex parts). There was a small amount of alignment necessary in orderto collect reflected light in both polarization channels from this closerange. Electronics and computer were placed on the horizontal strutbehind the lidar system. The scanner 1304 allows the lidar 1302 to moveup, down, and across the entire river scene which encompasses ahorizontal size of 45 cm×500 cm.

Referring to FIG. 13F, the flowing river scene 1306 is shown. The riverscene 1306 was constructed in a channel configured to mimic a flowingriver. The channel had a dimension of height 20 cm, width 50 cm, andlength 800 cm was constructed from wood having a bottom 1334 made fromplywood, first side wall 1332 made from 2×4s, a second side wall 1336made from 2×4s. A 5 cm-high dam was arranged on a lower end of thechannel causing a slight reservoir to build up, causing the water depthto increase as the water flowed downstream. The whole flume was tiltedat 1.25 degrees.

Water was added to height at a first end of about 7 cm and second end of18 cm and circulated from the lower part of the flume back up to theupper by a pump at a flow rate of about 1 cubic foot per second. Anumber of rocks 1322 and other objects are placed indiscriminately onthe bed of the channel to provide submerged objects to detect. Shown inthe bottom left of this image is half of a Nerf football 1340 that isattached to the bed surface. In the middle of the image is aconcentrated single-layer bed of rocks 1322. A tape measure runs alongthe side of the flume in order to verify horizontal distancemeasurements.

The flowing river scene was configured a small indoor river (flume) usedto investigate how sediment and underwater structures affect and areaffected by flowing water. Different size rocks were placed into theriverbed to provide submerged objects to detect, and then flowing waterwas circulated through the flume creating a rough water surface. Rock1316 was a whitish in color having a dimeter of about 10 cm to about 15cm, rock 1318 was greyish in color having a dimeter of about 10 cm toabout 15 cm, and rock 1320 was reddish in color having a dimeter ofabout 10 cm to about 15 cm as shown in FIG. 13D. Rocks, 1316, 1318, and1320 were oriented in a triangular fashion in the stream. These rockfeatures 1316, 1318, and 1320 served to create a turbulent flow whilewater was running in the flume and also served as objects to identifyand measure with the lidar system.

A bundle of rocks 1322 each having a diameter of about 5 cm and twolarger rocks 1324 having a dimeter of about 10 cm to about 15 cm wereoriented in the middle portion of the stream as shown in FIG. 13E. Thelarger rocks, 1324, were used to anchor the smaller bundle of rocks1322.

In this example, the mapping was conducted by the lidar system describedwith reference to example 4. The system was operated by one person andthe data collected using a Windows PC laptop. The lidar was turned onprior to operating the scanner and operated in the same configuration asin Example 5 with two differences: 1) the Lidar was attached to the scansystem via a custom aluminum mounting plate 2 feet above the flowingwater, as shown in FIG. 13B, and 2) the detector for the cross-polarizedchannel was replaced with an APD (ThorLabs APD430a). The scan system wasoperated to perform repeatable motions up, down, and across the flume.

The lidar time-of-flight (TOF) data for each channel were adjusted fordifferences in optical and electrical timing paths by illuminating asolid target below the mounted lidar. In this example, the plywood bed,before water began to flow, was used. By accounting for this timedifference, the two lidar receiver channels recorded the same TOF to theplywood bed. Water began to flow through the flume. Lidar TOF data fromthe surface measurement and from the bottom measurement were differencedand the index of refraction of 1.33 was applied to determine depth usingequation 8. The scanner x-y position was referenced to the lidar depthestimate using the scanner encoder data and a common computer time. Thelidar depth estimate with the corresponding computer time was thenassigned to the x-y position on the flume. One example of a scan patternresulted from the following steps: 1) start at the downstream left edgeof the flume 2) move the scanner upstream for about 700 cm, 3) step 1 cmacross the river, 4) move the scanner downstream for about 700 cm, 5)step 1 cm across the river, 6) repeating steps 2-5 until the entireflume crossed the entire flume. An output from combining the lidar andscanner data is shown in FIG. 13C, referring to this figure it is shownthe derived water depth over the extent of the flume illustrating depthin centimeters and the cross-flume and down-flume is given incentimeters.

FIG. 13G illustrates a graph with an x-axis in [seconds] and y-axis in[cm] showing range to bottom with no water. More specifically, the graphshows a range to a single location without water present as a functionof time. Over the course of 1 second roughly 8000 laser pulses arefired. The shot-to-shot standard deviation is 5 mm. The individual lasershots 1342 are depicted and a rolling average 1344.

FIG. 13H illustrates a graph with an x-axis in [seconds] and y-axis in[cm] showing range to bottom through flowing water at flowrate of about1 cubic foot per second [cfs] More specifically, the graph shows a rangeto a single location flowing water as a function of time. Over thecourse of 1 second roughly 8000 laser pulses are fired. The individuallaser shots 1346 are depicted and a rolling average 1348.

FIG. 13I illustrates one-dimensional map of a flume with an x-axisdownstream position [cm] and a y-axis height [cm]. More specifically, atop surface and bottom surface from ten consecutive sweeps up and downthe center of the flume resulting in the plots shown here in the blackdots. Line 1352 shows the average measurement of the bottom surfacewhile line 1350 shows the average measurement of the top surface of thewater. The line 1352 shows the rocks and other objects distributedthroughout the bed of the flume. Due to the nature of the water flow andthe setup of the scan tracks, the water surface appears to move closerto the lidar as the lidar scanned in the downstream direction. This isdue to the increased water depth as the water traveled downstream.

FIG. 13J illustrates a depth of the flume with a x-axis downstreamposition [cm] and a y-axis depth [cm]. More specifically, individualmeasurements of the depth from ten consecutive sweeps up and down thecenter of the flume result in the plots shown here in the black dots andline 1354 shows the average of all ten sweeps.

The results from the testing demonstrate that the lidar system andtechnique works in a variety of shallow water conditions and submergedobjects with about a 1 cm by about 1 cm horizontal resolution, dependenton scan parameters, and yielded less than about 1 cm verticalresolution, thus providing an unprecedented detailed estimate of waterdepth and subsurface morphometry. Ranging to the top and bottom surfaceswas consistent throughout the test and was possible through the flowingwater surface, as shown in FIGS. 13B and 13D.

Example 7

This example illustrates a lidar system configured to map the height ofat least two surfaces in a high flowing indoor river flume with wavysurfaces. The experimental setup used was a lidar system 1402 describedwith reference to Example 4 positioned over a fast flowing shallowearthen embankment as shown in FIG. 14A and FIG. 14B.

Referring FIGS. 14A and 14B, the setup 1400 includes a lidar system 1402arranged over tank 1404 on a rail system 1406 having a volume of about50 cubic meters with an earthen embankment 1408 of a sand-clay-gravelmixture with a volume of 12 cubic meters and a mass of about 30,000 kg.In operation, a flume 1410 of flowing water flows over the earthenembankment 1408 at a flow rate in a range of about 0.1 [cfs] to about 10[cfs]. The rail system 1406 suspended along the longitudinal length(about 6 meters) over the flume 1410, and the downward-pointing lidarsystem 1402 attached to a sled on the rail system 1406 and can be pulledback and forth along the rail.

This example is configured to demonstrate the utility of a shallow-waterlidar system for surveying an overtopping erosion test under runningwater conditions. The lidar system offers a new technology that canprovide added research capability in the field of surface andovertopping erosion. Specifically it provides the ability to measure thewater depth (even for shallow water) and bottom surface erosion withhigh flowing water present.

In operation, the lidar system 1402 was calibrated by operating while nowater flow was present. The lidar illuminated a flat section of the dryearthen embankment and recorded time-of-flight (TOF) data of the laserpulses for the co-polarized and cross-polarized detectors. Anydifference in time due to optical and electrical path differencesbetween the two detector channels is determined by taking the differenceof the TOF estimates for the two detector channels. This self-calibratesthe system so that semi-transparent measurements of two surfaces or morecan be associated with the range difference between surfaces. Thesled-mounted lidar on the rails was manually pulled along the rails fromone end of the flume to the other at a rate of about 4.0 feet persecond. This took less than 5 seconds per pass in one direction over alength of about 20 feet. The lidar system 1402 was pointed downward andthe height from the earthen embankment ranged from about 3 feet to about10 feet. The water depth varied with flow rates but ranged from lessthan a centimeter to tens of centimeters in different sections. Thelidar was operated with the same configuration outlined in Example 6.The lidar was turned on prior to water flowing and remained onthroughout the duration of the experiment. The data acquisition systemrecorded backscattered laser light in both receiver channels per laserpulse (8 kHz laser repetition rate) while the lidar was pulled along therail. A real-time display on the computer provided an indication oflidar signal acquisition. A rangefinder (make: Laser Technology, Inc.,model: Universal Laser Sensor) was mounted on the sled and alignednormal to the lidar optical path to provide position informationrelative to a target located at the end of the flume. The rangefinderdata recorded at 100 Hz and was synchronized to the same computer clockas the lidar. Therefore, the location of the lidar measurement along theflume can be determined by the computer time that data was collectedwith the rangefinder.

FIG. 14C illustrates a graphical representation of 10 [CFS] flow ofwater on a bed surface downstream scans with 2 cm horizontal resolutionvertical displacement related to time between scans having an x-axisdistance [m] and y-axis relative height [cm]. This graph furtherillustrates unprecedented precision measurements of shallow watersoil-eroded surface under flow conditions ranging from 0.1 [CFS] to ashigh as 10 [CFS]. The graphic depicts horizontal distance on the x-axisin meters and represents the length of the flume in the longitudinaldirection [m]. The y-axis provides a relative height measurement fromthe lidar to the earthen embankment in [cm]. Multiple longitudinalprofiles of the embankment are provided during high flow conditions ofabout 10 [CFS], and the longitudinal profiles are offset vertically. Thelidar results illustrate the capability of providing high-precisionlongitudinal profiles of the earthen embankment at 2 [cm] horizontalresolution over a 6 [m] span, and it can provide a single profilemeasurement in less than 10 [sec], with the potential for even fasterscans if supported by a properly advanced scanning system.

Each 2 [cm] horizontal bin from a single longitudinal scan oftencontained 50 or more laser pulses producing a mean range estimate to theembankment with about +1 [cm] precision. Aggregating consecutive scansimproved the precision of the mean embankment position relative to thelidar to even better than 1 [cm]. This measurement fidelity is muchbetter than other lidars can produce, and this level of fidelityprovides a better overall understanding of the likelihood of breach aswell as a better estimate of breach initiation time, breach formationtime, and peak breach outflow. The depth measurement has been aconsistent feature of the lidar demonstrated in previous tests.

Example 8

FIG. 15A illustrates the data from the lidar system according to Example7. The data is shown in a graph having an x-axis showing horizontaldistance [m] and vertical distance [m] in the graph data is binned in 10[cm] height bins and 1 [m] distance bins, and the number of lidar hits,anywhere from about 0 to about 50, in each bin is plotted in the contourplot.

FIG. 15B illustrates an overhead view of the terrain 1500 covered inFIG. 15A, with the flight line 1501 across the image, a grove of trees1502 and house 1506, correspond to data 1504 and 1508, respectively.

In this example, an aircraft was flown from left to right (east towest). On the left portion of the picture is a strand of trees and onthe right side is a small red-and-white structure surrounded by achain-link fence. The aircraft included a lidar system as described withreference to example 1. The aircraft was a Boeing DC-8 at a speed ofabout 90 [m/s] and elevation of greater than about 500 [ft]. The lidarwas mounted over the nadir viewing port number 5 on the DC-8 using theexisting baffling system and window. The mounting system for lidarsystem was used that met the DC-8 standards and provided sufficientvibration damping for the lidar system. This lidar system did not havescanning capability. This example served as a verification step tocomplete integration on a mobile platform. The lidar system may be usedwith a communication system as described with reference to FIG. 10 .

In this example, polarization discrimination was a component of themeasurement technique employed, and thus the transmit window in thenadir view port number 5 must be polarization preserving. The initialwindow selected for us to use was made of BK7 glass with a MgF₂ coating,and was tilted by approximately 1.5 degrees to facilitate control ofreflections. A simple test, where laser light was transmitted to acontrolled area on the hangar floor, showed the MgF₂ coating to havepoor transmission at 532 [nm] and introduced a retardance to thetransmit polarization, thus hindering the polarization sensitivity ofthe lidar. The one way transmission for the BK7 window, at 532 [nm], wasmeasured to be approximately seventy two percent (72%), and thealteration to the polarization was determined to be more than 10 degreesof rotation.

The window on the hanger floor was changed to a custom NASA-made fusedsilica window. The transmission of the fused silica window was measuredto be approximately ninety three (93%) at 532 [nm] with no discerniblealteration to the polarization. Both of these measurements were donewith the instrument mounted in the aircraft and diagnostic equipmentlocated under the port window. A baffling system, metal tube extendingfrom the instrument to the window port, was used to isolate the transmitportion of the lidar system from the receiving telescope, to eliminateany stray laser light that could contaminate the received backscatteredground signal. This helped control the back reflections of the laserlight from the window.

The lidar operated on the aircraft during flight with one operator onboard to cycle power and monitor progress. Flights were made primarilyover land and the lidar was powered on once 500 feet altitude wasreached. The data was collected on both receiver channels as describedin previous examples.

The data from this example demonstrates our ability to detect smallobjects and variations in terrain from fast moving airborne platforms.The lower portion of FIG. 15B shows an aerial photograph of the scenewhile the upper portion, FIG. 15A, shows a height profile of the scenederived from our Lidar data. Two features in particular stand out. Thefirst is a band of trees on the left side of the FIG. 15B. Aircraftnavigation data were provided and the altitude information, aircraftattitude, and velocity were used to reference the lidar data to theground.

Referring to FIG. 15A, lidar data from the treed region 1504 is shown inthe left side of FIG. 15A and demonstrate mapping both of the treecanopy and of some sub-canopy structure. Based on the location of theground on either side of the trees (just below 0 [m] height), a few ofthe lidar returns made it through to ground level during the overflight.The right side of FIG. 15A shows the structure 1508 sitting on top of asmall hill. The structure appears to be roughly 2 [m] tall. Thechain-link fence shows up on the left and right sides of the structure.

With our lidar system we can measure the height of the canopy whilesimultaneously detecting leaves and branches below the canopy. Somelidar pulses also reach the ground level through this region. Measuringsub-canopy lidar returns is a morphometric measurement that allows us todetermine that we are hitting trees rather than a small hilly region. Onthe right side of the image is a small building situated on a slighthill and surrounded by a fence. These objects are all clearlyidentifiable in the lidar data shown in FIG. 15A. This engineering testallowed us to demonstrate operation from a mobile platform as well as atest of the morphometric capability provided by the lidar system in aterrestrial setting. Vertical resolution of a few centimeters wasachieved with 10 [cm] horizontal resolution.

Example 9

FIG. 16 illustrates a picture of an office building measured with lidardepolarization ratio according to example 8. In this example, a lidarsystem included the same configuration as example 1. The lidar systemwas operated by one person and placed in a fixed position across thestreet from the building to record data on both the co-polarized andcross-polarized channels. The ratio of these two signals provides whatis called the depolarization ratio and allows for surfaces withdifferent polarization scattering characteristics to be identified. Apolarization calibration technique is performed before making theobservations. Here, the laser light leaving the transmitter is passedthrough a half-wave plate and illuminates a hard target of knowdepolarization. In this case a piece of glass whose depolarization ratiois less than 0.05. The reflected signal from the glass was recorded bythe lidar receiver in both the co-polarized and cross-polarizedchannels. This allows any differences in polarization caused by thesystem to be determined and accounted for when observing targets.

Once data in one position was recorded, the lidar system wasrepositioned and new data was collected in the same manner. The locationof the laser spot on the office building was identified by eye and thetime of the data collection was recorded in a lab notebook. Thecollection of data represented a sampling of different surfaces on thebuilding and identified by their differing polarization characteristics.

Referring to FIG. 16 , the picture is of a local office building 1600with the measured lidar depolarization ratio shown in the scale color1602. In this example, it is shown that the lidar depolarization ratiocan be used to classify building materials. Note that the lidar returnsfrom the glass show up with very low depolarization ratio 1604 while thebricks 1606 and other objects in the scene exhibit higher depolarizationratios, approaching unity.

The lidar allows for precise range determination to underwater objectsand can simultaneously record information about the intensity ofscattered returns. The polarization discrimination of the technique usesthe relative intensity between the two polarization channels to allowfor the classification of objects based on their depolarization ratio.Natural and man-made objects depolarize scattered light dependent on themicro-properties of the material, e.g. whether it is rough or smooth orwhether the material is a dielectric or a conductor. By recording thedepolarization ratio we can classify a target. This is shown in FIG. 16. This single-axis characterization of the material is limited but doesprovide valuable information as to the microphysical properties of theobject. For instance, in FIG. 16 we can discern the difference betweenscattering from glass and from brick, and we can even observedifferences between two types of brick (shown as light and dark in theimage, and as red and yellow/green/blue in our depolarization spots).This scatterometry measurement can differentiate between materials andobjects that may prove useful in autonomous vehicle advancements.

Although the present disclosure describes components and functionsimplemented in the aspects, embodiments, and/or configurations withreference to particular standards and protocols, the aspects,embodiments, and/or configurations are not limited to such standards andprotocols. Other similar standards and protocols not mentioned hereinare in existence and are considered to be included in the presentdisclosure. Moreover, the standards and protocols mentioned herein andother similar standards and protocols not mentioned herein areperiodically superseded by faster or more effective equivalents havingessentially the same functions. Such replacement standards and protocolshaving the same functions are considered equivalents included in thepresent disclosure.

The present disclosure, in various aspects, embodiments, and/orconfigurations, includes components, methods, processes, systems and/orapparatus substantially as depicted and described herein, includingvarious aspects, embodiments, configurations embodiments,subcombinations, and/or subsets thereof. Those of skill in the art willunderstand how to make and use the disclosed aspects, embodiments,and/or configurations after understanding the present disclosure. Thepresent disclosure, in various aspects, embodiments, and/orconfigurations, includes providing devices and processes in the absenceof items not depicted and/or described herein or in various aspects,embodiments, and/or configurations hereof, including in the absence ofsuch items as may have been used in previous devices or processes, e.g.,for improving performance, achieving ease and/or reducing cost ofimplementation.

As the foregoing discussion has been presented for purposes ofillustration and description, the foregoing is not intended to limit thedisclosure to the form or forms disclosed herein. In the foregoingdescription for example, various features of the disclosure are groupedtogether in one or more aspects, embodiments, and/or configurations forthe purpose of streamlining the disclosure. The features of the aspects,embodiments, and/or configurations of the disclosure may be combined inalternate aspects, embodiments, and/or configurations other than thosediscussed above. This method of disclosure is not to be interpreted asreflecting an intention that the claims require more features than areexpressly recited in each claim. Rather, as the following claimsreflect, inventive aspects lie in less than all features of a singleforegoing disclosed aspect, embodiment, and/or configuration. Thus, thefollowing claims are hereby incorporated into this description, witheach claim standing on its own as a separate preferred embodiment of thedisclosure.

Moreover, though the description has included a description of one ormore aspects, embodiments, and/or configurations and certain variationsand modifications, other variations, combinations, and modifications arewithin the scope of the disclosure, e.g., as may be within the skill andknowledge of those in the art, after understanding the presentdisclosure. It is intended to obtain rights which include alternativeaspects, embodiments, and/or configurations to the extent permitted,including alternate, interchangeable and/or equivalent structures,functions, ranges or steps to those claimed, whether or not suchalternate, interchangeable and/or equivalent structures, functions,ranges or steps are disclosed herein, and without intending to publiclydedicate any patentable subject matter.

The headings, titles, or other descriptions of sections contained inthis disclosure have been inserted for readability and convenience ofthe reader and are mainly for reference only and are not intended tolimit the scopes of embodiments of the invention.

What is claimed:
 1. A method of determining characteristics of at leasttwo surfaces, comprising: transmitting a pulse of polarized energy tothe surfaces; receiving reflected energy from the surfaces received overa period of time after a transmission of the energy; sensing informationindicative of one or more properties of two or more portions of receivedreflected energy, wherein the properties of each of the portions of thereceived reflected energy comprises one or more of: (a) an orientationof the portion, (b) an angular spread of the portion, (c) a degree ofpolarization of the portion, (d) an azimuthal polarization pattern ofthe portion, (e) a high order scattering profile of the portion, and (f)a range of a respective property being one of the properties (a) to (e);and determining, using computational equipment, one or more elapsed timeamong the two or more portions of the received reflected energy, basedon one or more differences among the properties of the portions, whereinthe elapsed time is less than a duration of the pulse of polarizedenergy.
 2. The method of claim 1, further comprising determining therelative distances based on the elapsed time.
 3. The method of claim 1,further comprising determining one or more of the properties based onthe data.
 4. The method of claim 1, wherein the properties of each ofthe portions of the received reflected energy comprises (a) theorientation of the portion.
 5. The method of claim 1, wherein theproperties of each of the portions of the received reflected energycomprises (b) the angular spread of the portion.
 6. The method of claim1, wherein the properties of each of the portions of the receivedreflected energy comprises (c) the degree of polarization of theportion.
 7. The method of claim 1, wherein the properties of each of theportions of the received reflected energy comprises (d) the azimuthalpolarization pattern of the portion.
 8. The method of claim 1, whereinthe properties of each of the portions of the received reflected energycomprises (e) the high order scattering profile of the portion.
 9. Themethod of claim 1, wherein the properties of each of the portions of thereceived reflected energy comprises (f) the range of (d) the azimuthalpolarization pattern of the portion.
 10. The method of claim 1, whereinthe surfaces comprises two surfaces, wherein the portions comprises twoportions, and wherein the elapsed time comprises one elapsed time. 11.The method of claim 1, wherein the pulse of polarized energy istransmitted by a transmitter comprising a laser being one of a polarizedlaser, a pulsed laser, and a continuous wave (CW) laser.
 12. The methodof claim 1, wherein the received reflected energy collection is receivedby one or more sensors configured to receive a portion of the receivedreflected energy.
 13. A method of imaging skin of a patient, comprising:transmitting a pulse of polarized light to the skin of the patient;receiving a collection of light, wherein the collection includes lightfrom reflected from at least two portions of the skin of the patientover a period of time after a transmission of the pulse, wherein thefirst portion has a first depth and a second portion has a second depththat is greater than the first depth; sensing information indicative ofone or more properties of two or more portions of received light in thecollection, wherein the properties of each of the portions of thereceived light comprise one or more of: (a) an orientation of theportion, (b) an angular spread of the portion, (c) a degree ofpolarization of the portion, (d) an azimuthal polarization pattern ofthe portion, (e) a high order scattering profile of the portion, and (f)a range of a respective property being one of the properties (a) to (e);and determining, using computational equipment, characteristics of theskin based on one or more differences among the properties of theportions, wherein the elapsed time is less than a duration of the pulse.14. The method of claim 13, wherein the determined characteristics ofthe skin include characteristics indicative of normal skin pathology.15. The method of claim 13, wherein the determined characteristics ofthe skin include characteristics indicative of abnormal skin pathology.16. The method of claim 13, wherein the transmitting a pulse ofpolarized light comprises transmitting with a laser selected from thegroup consisting of a polarized laser, a pulsed laser, and a continuouswave (CW) laser.
 17. The method of claim 13, wherein the receivedreflected energy collection is received by one or more sensorsconfigured to receive a portion of the received reflected energy.
 18. Anavigation system, comprising: a light transmitter configured totransmit a pulse of polarized light to at least two surfaces within atransmission path of the pulse; a light receiver configured to receive acollection of light, wherein the collection includes light from thesurfaces received over a period of time after a transmission of thepulse by the light transmitter; a sensor system configured for sensinginformation indicative of one or more properties of two portions ofreceived light in the collection, wherein the properties of each of theportions of the received light comprise one or more of: (a) anorientation of the portion, (b) an angular spread of the portion, (c) adegree of polarization of the portion, (d) an azimuthal polarizationpattern of the portion, (e) a high order scattering profile of theportion, and (f) a range of a respective property being one of theproperties (a) to (e); and computational equipment configured fordetermining an elapsed time between the two portions of the receivedlight, based on a difference between the properties of the portions, anda relative distance based on the elapsed time, wherein the elapsed timeis less than a duration of the pulse.
 19. The navigation system of claim1, wherein the navigation system is arranged in an automobile andconfigured for autonomous driving or to aid with driving.
 20. Thenavigation system of claim 19, wherein the computational equipment isarranged in the automobile.